Re the Coroplast, I have access to plenty as my wife, a flower arranger, uses it for making backboards to drape, etc. But my problem, carrying painting gear, food and water on day trips is solved in my low-crime area by the Basil Cardiff pannier baskets, which are open topped, not suitable for tourers who don't know what they will find when they arrive.
However I need space for the usual small valuables like wallet, camera, permanent small watercolour kit (postcard size), which is currently served by the tiny "panniers" on the sides of German-made (Topeak, probably) rack top bag the man purpose of which is to hold the huge battery for the bike's electric motor.
Before this I used ladies' large leather bags tied to the saddle across the bike or lengthwise on the rack, usually with their own straps. These bags needed stiffening on the bottom and shaping around the side or they'd sag -- ugly! -- which isn't good for fine leather and rather spoils the look of the bike. For this at first I used Coroplast, but I was soon fed up with the space the stuff took, and its stubborn refusal to shape itself without considerable advance planning and work. The solution came from my sketchbook practice.
The way I make a sketchbook is to buy a cheap display book, rip out the display sheets and throw them away, and cut the semi-stiff thin P-something cover to size. My sketchbooks are not sewn, the sections (technical term for a large sheet folded into smaller ones in the immediately pre-sewn stage) are just held into the existing spine of the cut-down display book by tooth floss cord from the chemist, because it is thin and strong and pre-waxed and immediately available, which is not "sewn" but simply knotted around the P-something's semi-stiff plastic spine. The made book is then slipped into a reusable leather slipcase to look smart, and not fixed in there either, just held by the inner flaps, loose to slide from side to side though cut more closely to size up and down for technical en plein air painting reasons. The inner books don't even have to be made very neatly because the outer leather slipcases designed by couturiers hide rough work and slip-ups; other painters are always oohing and aching over my sketchbooks. These books have amazing articulation (important to get a flat surface across the fold or even on a single page to paint on), and are very durable.
So, since I already had some large display books bought at the pound shop for making large sketchbook inner covers, as above, I tried this thin, stiff for practical purposes, lasting, material as bottom and inner side surrounds on the smart bags made in relatively thin leather which were permanently on the bike. This worked brilliantly for years until I needed that space for the bigger battery.
The next question that arises is: Can the installation be made as neat as the one Mickeg shows in Coroplast, specifically as regards folds pre-scored into the stiffener. And the answer is yes, it can. The neatest folds will need either a wheelie on a rotary knife or guillotine to cut a dashed line as a hint where it should bend, or a very shallow V-cutter to remove some material, or a hot knife attachment to your soldering iron to score a line by heat. In my sketchbooks, though I have all the equipment mentioned, I don't bother: I use the excellent scores in the commercial production and when I want a thicker or thinner book than the original spine of the display book, I cut through the middle of the spine and remove or staple or glue in a flat piece to make the width desired, thus reusing the production folds.
On my bike I don't even go to that bother. I cut a flat piece to size for the bottom of the bag, then cut a piece to height and length to form a vertical space with the unfixed ends overlapping for articulation, and let the weight of the contents push the P-something into shape. The bottom and the sides are not attached to each other either. It's admittedly rougher than Mickeg's tidy work, but very likely more flexible in packing* and definitely more reshapable out on the road. And even when not maxed out with custom-fitted folds, the amount of space recovered over Coroplast is obvious and well worth having.
*I once wanted to carry a camera with a long lens crosswise in the bag surrounded by all kinds of other stuff that wouldn't be damaged and would protect the lens but the camera and lens were too long and I didn't want to split them because then they would have to be cased, for sure, and go in a car or on their straps across me. What I did was cut a round hole in the saddlebag stiffening for the lens, put the camera body between the outside leather of the bag and the stiffening, with the lens through the hole into the bag, and then simply shoved my sketchbook between the LCD on the back of the camera and the leather of the bag to protect the electronics from being bashed. Worked well. A flat piece of the P-something over the LCD would have made a neater package but the party was ready to ride at my door when I discovered the camera, which was the point of the expedition, wouldn't fit in its least exposed position, and I was damned if I would run up four floors to fetch either another piece of the P-something or the clumsy cases from my study, and wear the hefty thing (yeah, I know, I should have bought the Olympus or Pentax gear instead of the Nikon) around my neck for hours. So, out came my trusty camping knife, spoon and fork folder, and a suitable hole was cut on the spot. Some people who were on that ride are still under the impression that I'm quite handy with tools -- not so; I'm just a lateral thinker, and like most here, I prefer orderly planning ahead to bodging under pressure.
