I have always used plumbers putty, But it falls out and gets thinner as the mold gets hotter. Some of my molds I dont want to permanently block off hooks, weedgaurds etc... I ordered a pound of this stuff to try. I think it's expensive, But think it will work great.
https://www.rotometals.com/casting-r...putty-1-pound/
This is a 1 pound chunk of Babbitt Casting Putty and works for temperatures up to 950 Degrees F. For use as damming compound for pouring molten Babbitt, vibration suppression, positioning delicate parts, holding solder while soldering wires on generators, and aids in positioning during welding/brazing. It has extreme temperature resistance, is easily and quickly Removed, remains flexible and holds tight, and it is easily packed in place.