Making Conforming Dies with Devcon for a hydraulic press

I just tried Jett Ballistic to make an impression die.
I was able to get a pretty good looking impression from an old button, but when I tried it in my 12 ton press with annealed 24 guage cooper , it just mushed out the details & the impression itself spread a little wider.
So, my question is , does it need a containment to work ? Or am I wishing for things that won’t work at all ??
Patty