Disclaimer: I have never seen or built a donkey and I don't know anything about logging, On30 or 303's, so I am likely the least qualified person to answer your post. I do know how to club away at the keyboard, though, so I'm gunna try
Referring to the famouse shamos 303 donkey here, the one in the Academy: If I'm not mistaken, a 303 casing would be .3" at the top where it was once crimped to a lead pill. Just earballin it, I would say it's about 5 times taller than that, or about 1.5" Sooooo, if I'm not mistaken, I guess you about double it to get O from HO, so I guess you want it to be about 1/2 in dia or maybe 3/4 and about 3" tall. Someone please correct me if I'm wrong, but this should get you close enough for government work. I would start with 1/2" PVC pipe and go from there
Maybe even 1/2" PVC caps, which are about 5/8 or 3/4 dia, then go to 1/2" pipe at the top?