In article ,
"Chris Wilson" wrote:

"Obviously, any filler that expands, heats up, or
dissolves plastic is out."

20 minute casting plaster! It stays cool, doesn't expand, doesn't harm
plastic, and it's cheap...

I find that plaster does gets warm. It's more noticeable with large
castings. But it doesn't get as warm as casting resin, and not enough to
harm a plastic model. Plaster also tends to cool down quickly after a
certain point, probably due to evaporation of the water.