I've been meaning to make these available seemingly forever now.
I use these sturdy little, lightweight, low density off cuts from making things to elevate my drop rings in the kiln.

(9) 1/2" ceramic risers. (.44"-.52")

Stack 'em in combination with each other to have more control over your drop height. Sold in multiples of three because there's no need for four legs under your mold. :-)

Note: Thickness of just about every material I order seems to be variable these days. These pieces are no exception but I do my best to keep thicknesses consistent within individual packs. They can always be easily thinned on a lap wheel or even using wet dry sandpaper on a flat plane. (I do it all the time. I also just ignore the tiny differences in thickness occasionally)