Green florist foam is great!

I used the green florist foam that is used for floral arrangements to
make a large one time use mold. The foam cuts with an extremely smooth
surface. You can take huge hogging cuts at 80ipm. Dust is minimal and
foam doesn't dull cutter.

I built up my 12X26X3.5 inch block by gluing individual bricks
together with hot "bag wax" (melted wax+shopping bags) glued to a
sheet of blue insulating foam. After the block was machined I coated
the entire surface with wax and repeated my finishing passes. Surface
came out smooth, hard and dense. Mold surface is suitable for resin or
plaster casting. I guess it could also be used for Rick-O-Matic
aluminum/foam casting.

Cost from Michael's craft supply about $4.00. Amazing stuff.

Roger