My pops used to make them outa 2" pvc elbows n a plastic saucer plate, semi truck valve stem n jb weld. Worked fine for many yrs.
U dont hav to use anythng to take up the gap from the runflat. Iv ran hmmvs with out them with now issues. I hope u know how to get that runflat out. Iv put them in n its a sever pain in the butt. Use a 10k ratchet strap n fold it in half n use a LOT of lube