I'm happy to hear you men ride clean bikes -- presumably the ladies are so used to having clean bikes, they don't brag about it.
I'm perfectly happy with the Chainglider, and if I somehow broke mine, I'd fit another one. I don't think I'll wear it out; the one now on my bike has travelled north of 6K and doesn't show any signs of wear, and I'm keener after the Chinese virus to reacquaint myself with well-loved lanes than reaching for far horizons, so new dangers would be unlikely and extraordinary.
However, rereading this thread, a couple of ideas do occur to me:
A. A MACHINED PLASTIC GUARD
A common way for DIY makers of fountain pen barrels and colourful, distinguishing tool handles is to have their wives collect plastic bottles and containers, to melt them down in a rectangular cake pan, then give the resulting goo a small stir until it looks like marbled paper, and leave it to set. This recycled plastic is superb to saw, carve, lathe, mill, glue, paint, and wears remarkably well for a material so easy to machine. I haven't done the melting part myself, though I'm into fountain pens, because my skills on my toy lathe (of a size to make watch movements) aren't good enough to entrust expensive or irreplaceable pen components to, though I've made a couple of handles for doll's house size lathe tools. I just commission the parts I want from people who make them routinely. I think a long enough piece of this stuff could be machined out so that it is solid and, if supported in the middle, stiff enough even for a tandem. Maybe it won't in the most workable state last as long as a Chainglider, which I think has carbon fibre or hard rubber in it, but you could sacrifice some of the workability by putting a cotton cloth over a former and pouring the molten plastic across it, and then machine the outside, which will be easier to finish to an acceptable standard than Dan's hard work with the spatula and gloved hands on hot plastic. (! See below about a similar but less dangerous problem in cold molding wood !) This cloth plus plastic (or resin) material is called mikarta when it is made into knife handles for Real Men <TM>. Did I say cheap yet? Actually a saving, as you don't have to pay to have the surplus plastic packaging removed.
B. MOULDED WOOD, AN OVERLOOKED LIGHTWEIGHT STRUCTURAL MATERIAL
Back in the 1960s, after wrecking several transocean glassfibre yachts worth in today's money more millions than I'll ever see again, alas, it was suggested to me by an old racer, who wanted me to keep competing against him, that I build a yacht from wood ''if you don't want to drown." He was also in a position to give me as much superior quality naval wood of any type I could desire entirely free of charge, treated any way I wanted it, and expert advice with it. After speaking to several naval architects who'd already tried their level best to drown me, who anyway had the same initial reaction as I had, What the devil is this old chappie on about? but, unlike me, weren't willing to move on from reliance on the unproven material GRP (FRP for the Americans), I designed the yacht myself out of Hereshoff's book, and built it by parking a container load of beer (at the time my ad agency had lent me to a brewery) in the shed and telling my mates my shipbuilding experience was happy hour; you don't build a 68ft transocean racer and circumnavigator by your lonesome in any reasonable time, and mine had be built, outfitted and shaken down for a race at a fixed time only 11 months away. The hull itself was built over a mound of earth with the only solid wood in the entire thing on top (to mount the keel, the auxiliary engine and gennie, and to anchor the mast), covered diagonally with two layers of sacrificial laths of wood, and then we cold-molded overlapping 6in squares of wood veneer over the laths, tacking them down with electric window dressers' staplers using copper staples because, while we would later nip off the staples on the inside, with a tool I designed to nip and form the predetermined excess into a rivet head, the rest of the staple would be between layers and unreachable. When the underpart of the hull was finished, we turned the whole thing over with the aid of a helicopter, dropped it in a hole in the earth to brace it, and proceeded in the same way to make the deck integral with the lower shell. After that a couple of engineering chums from HP used FEA to calculate that even the biggest sea wouldn't ever flex it. Bulkheads for attaching stuff to rather than for strength were made the same way, all temporary support structures (some of it nothing more than foam) was pulled out and thrown away (and, trivia, the voids had to be filled with foam again when the ship was repurposed after its racing career as a luxury sunshine cruiser because the slightest slap of water on the hull was magnified into a tsunami in the voids, which was what much later made me think I could be a loudspeaker designer...). Because I'm a belt and braces man, and had already learned that the survivors in that sort of racing took care of every detail, we gave all parts of the hull regularly in contact with anything a couple of extra stepped layers on the inside, ditto all angles that would be under stress. Several engineers on the team thought I was crazy to add weight to a racing craft already so strong. The proof is in the pudding. The ship survived racing several times across the Southern Ocean, a rough sea at any season, multiple passages around Cape Horn, life at my floating dock in the rough sea where the Indian and Atlantic meet at Cape Agulhas, some notorious Indian Ocean monsoons, a decade or so as a luxurious rental in amateur hands, and is still providing solid security under junk rig for a family trading from Indonesia up into the South China Sea, not a place where you want to send amateurs in flimsy ships. I became a lifelong convert to the advantages of moulded wood. Or, if you guys want a British example, look up the wooden chasses under Jem Marsh and Frank Costin's early Marcos cars, of which many are still delighting drivers after 60 years or so. A chain guard (and how about mudguards to match?) would be a doddle in layers of wood veneer. In fact, an entire bike in layered wood veneer would need to be pretty incompetently designed and constructed to weigh more than a steel framed bike, and could easily be made both stronger and lighter than steel. The problem is of course that such intensive labour costs plenty. So about now, someone says, Yes, but what about compound curves?, and the answer is that you just work in smaller and smaller squares of veneer, diagonally overlaid, with vents cut into the veneer as if you're working with cloth; working diagonally across the curvature is in effect its own curve-fitting procedure. I regret to inform you that for cold molding you need to wear gloves or you'll lose the skin on your hands, which can be pretty painful.
