Sorry, I've been away for awhile (work, fall weather, etc...)
I would not use polypropylene or any other plastic for that matter (UHMW, PE, etc.) Teflon would be OK if you have lots of $$$
Phenolic is good for direct contact with the thermal die - it can take reasonable temps and is structurally rigid (it looks like wood in the pics of my early therm die sim). I am planning to use ceramic fiber insulation (poor man's Space Shuttle Thermal Tiles). I have various scraps of 1/8", 1/4" paper, 1/2" board and roll insulation left over from old projects. Glass wool or fiberglas insul could be used as "filler" between the therm die sim block and outer housing. Just some ideas...
Ceramic insulation
http://www.cotronics.com Great for insulating the thermal die assembly
>Flexible Ceramics >Blankets, Boards, Moldable sheets, Papers