Finding replacement bolts and lugnuts for trailer wheels is not terribly easy.  Most auto supply shops carry... automobile parts.  But with a patient sales associate that let me come in the back, I was able to rustle through the drawers and find ten bolts and nuts that matched the dimensions needed to replace the rusted out ones.  The nuts are a bit long, but I think it adds some character.

The floor is a sandwich of 3/8" plywood, 2x4's laid flat, and luaun plywood.  Everything has been glued, screwed, and urethaned at least twice.  Maybe three times on the bottom.  Still, I think another layer of protection might be a good idea.