My version of a mold for a Dremel/Flashforge and possibly other mk10 style hot-ends.
I use Red Permatex high heat RTV and it's worked quite well.
The trick I found that made all the difference is dipping both parts of the mold in diluted Dawn dish detergent before filling with the Permatex. Also waiting 24-48 hours so it can cure in that enclosed space.
Comes out very clean though you will probably want to let it cure for another 12 hours before you get it up to temp.
Designed in Tinkercad.