Details I didn't include on this prototype but would like to add in the future: 1) small sleeve tubes towards the rim end of the spokes (I might replicate these with styrene) and 2) a threaded air valve. I'm thinking of using a small bolt -- or maybe just a bit of brass tubing with some scratches to simulate threads.

On any finished wheels I'd also like to have some "permanent" mud. Any suggestions for making mud?