Looks like a chunk of wax to me....
How about improvising a mould by drilling a 2.25 inch hole in a thick chunk or stack of wood, putting a ball at the bottom to form the curve where the ball will be, and pouring wax?. YOu may have to make a dimpled base as part of the mould to center the ball and the rest of the lould.
If it is hard to get the ball out, just leave it and shoot as is.
You could cut the stack in half after you drilled the hole so that you could "open the mould" like a regular bullet mould to get the sabot out.
Parafin sounds cheaper than what your quote for UHMW (whatever that is...lol) and after you fire it, you could pick up the sabot (and the ball, of course) if you can find it and remould, reuse it.
No matter what you pick, watch the added weight and it's affect on pressure, of course