not a chainstay protector, but a guard/shield mounted inside the driveside chainstay near the rim to stop crap coming off the rear tyre onto the chain. i feel like i've seen something that does this but i cant recall where. any ideas?

I'd be making a shield from a tough waterproof textile like oxford nylon, maybe reinforced with wire sewn into the edges - a triangle zip-tied to the chainstay and seat tube, along the lines of a traditional skirt guard. Do you have a decent mudflap on the front fender? That's the most effective way of keeping road dirt and water off the chain IME.

front mudflap is a beast, which i guess highlights where the other road grime is coming from. i'll try a larger set of mudguards first, the ones currently fitted are pretty tight fitting and narrow.

https://vintagebicycle.wordpress.com...7/dsc_1644.jpg

You can see the rear half of the one I made out of an aluminium sheet and an L-bar in the video below. After a few years on the bike, I took it off because it was hard to maintain.

is that on the inside of the chain also?
I think I’m gonna use one of those fold-y front mtb guards, a mudguard stay shd pinch it for the lowest ring and hopefully it’ll flare out above that to keep the tyre crap off

No, it covered the outside of the chain. It had 3 segments: the rear extending up to the chainring and derailleur, a disk on the outer chainring, and an arc covering the front of the outer chainring. The goal was to protect the pants from chain staining. More extensive protection, including the rear, could be developed, but the problem could arise from gaps through which debris could enter and accumulate over time. Single-speed Dutch bikes use plastic shells that encase the entire drivetrain. Even if you obtained such a shell, it would limit the drivetrain you could use.

On that topic, on my folding bike in winter, I encountered wet snow filling the gaps between the 11-speed cogs, making them unusable except for the largest one. The snow was coming down into those gaps from the fender above.

try stuffing the pant cuff into your sock.

I have some plastic wheel skirts with holes zip tied around the wheels on my bike with a baby seat to keep the kid's blanket out of the spokes:

https://www.amazon.com/dp/B0C5DHRVLB

Could dip it in a pan of plastidip first, I suppose, to fill in the holes, and zip tie between the wheel and chain. There's lots of solid ones out there for specific cargo bike models, but very few generic.

I tried cargo nets first, but it's tougher to keep them out of the wheels and they stretch when someone tries to poke into the spokes, so I upgraded to these plastic ones.

OP's interest was in protecting the chain. My own strategy was to avoid fiddling with the attire, just having a bike that does not stain.

Reduce the agony. Simply put a chain guard on it

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