I've been thinking for a long time about the minimum weight solution for a spanner to operate the Rohloff gear shift in the event of a shifter / cable failure. I think I've found a good solution: A small 8mm 1/4" socket such as https://www.amazon.co.uk/gp/product/B0001P0WUM can be operated by a 6mm Allen key which is standard on any cycling multi-tool. Two flats on the Allen key engage with two sides of the 1/4" socket such that it won't slip.
(https://chainreactioncycles.scene7.com/is/image/ChainReactionCycles/prod10197_IMGSET?wid=586&hei=498)
This x-Tools set includes a socket with a stub on top which fits a 1/4in drive. I bought it specifically to get that useful tool, which I keep with another, much lighter driver and bit unit on the bike. (The original set pictured above is kept near my desk for repairs on small items.) Unfortunately, the one I'm showing is on the Chain Reaction Cycles site from outside Belfast, no longer available; I bought my set lightly water damaged for a huge discount but the alloy socket I wanted was fine and the rest cleaned up well.
British tool-makers/packagers Drapers used to carry and may still do so, a flat, lightweight open-ended 8x10mm wrench which they packaged in bike kits. I have one that came in a blue rollup toolkit which I've long since outgrown but the 8x10 spanner is in my Rohloff toolkit. You could check if any of the more traditional full-service bike stores like SJS maybe carries the spanner by itself. It's a decent tool, considering its low weight, but my next suggestion is a tool for the ages:
If you have access to a grinder of some kind, another option is to buy an 8x10mm brake spanner from your nearest motor factor, who'll have a range at various prices. They're all likely to be adequate for your light use. This is a spanner with high sides and only a small open end to minimize slippage. I ground mine flat over its entire length to minimize weight because at the time I intended carrying it on the bike, and it works well, but you could leave the 8mm side high because you don't want to round off an important nut like the one in the Rohloff, and just flatten the 10mm (on my daily bike the only 10mm nut requires a thinnish spanner). This is the one I actually used on the very few occasions I wanted to shift gears at the source, so to speak, to determine where gear 14 or 1 is when I forgot to roll the gear control all the way to the end before taking off the EXT box. It has its own place in my old pilot's case, now repurposed to my bicycle tools.