Kydex can be tricky. I found it worked better if my toaster oven was fully preheated for a while. If I put a piece in and turn the oven on, the piece was completely burned/melted in minutes. I put a mark on my oven where the kydex and holstex seem to be warm enough to work well, but not cooked, which is at like 310 degrees on my ovens dial. I prefer Holstex to Kydex, too. It has a different feel to it and I prefer the way it molds over kydex.
I am not sure if you can pre heat the thermo former or not, but that would be the first step and start off at a lower temp for the material you have and work your way up slowly. Kydex can get pretty hot when it's on the blade, so the molds need to be able to withstand that heat without slumping or melting as you found!