I have a 5kw CDH (chinese diesel heater), usually used to heat vehicles but I have it installed in my garage. There are two holes in the wall, one for air intake and one for exhaust. The exhaust air is very hot (I measured 300C even with exhaust wrap around it). I wanted a sleeve to go around it so that the hole in the wall is somewhat sealed and the exhaust wrap doesn't touch the wood of my enclosure or the OSB of the wall.

I designed a simple parametric mold which is two pieces. The idea is that you mix up food coloring, silicone caulking, and cornstarch. See the video below for more details:

Place the resulting goo into the bottom mold. The top mold is then placed on top and all the extra silicone mixture squeezes out. I applied clamps and came back an hour or so later to remove the cured silicone sleeve.

I didn't use any sort of mold release. The 5mm mold (version 1) I made survived without issue but the 3mm mold was destroyed.