Some time ago I found a cheap Ebay deal on 12"x12" sheets of Kydex
Kydex is a heat-moldable PVC plastic sheet. You can bake it at 250F for 2 minutes in a toaster over, then shape it pretty much any shape you want. Once it cools it retains the shape. People often use it to make gun holsters and other durable molded objects. I was able to build some special shoe inserts to relieve my fallen arches, in some custom shoes, but that's a whole 'nother blog
Shaping it involves either pressing a big piece of foam over the object, or using thick gloves to press it by hand. You can then make adjustments with a heat gun if the results aren't satisfactory. It is possible to weld two sheets together using a soldering iron, providing you can deal with the nasty fumes. I use a fume extractor fan to get them away from my face, probably should use more ventilation than that.
There are lots of other kinds of insulation that could serve. Fish paper
is commonly used, but I didn't think that was the best idea here. Heat Shrink is commonly used as well. I wanted to have a little better heat transfer out of the center of my batt, so I wanted to leave that area open. Heat shrink would trap any heat.
I liked the idea of having the BMS heat sink aluminum sticking out the end, and set directly against a metal (preferably aluminum) case to get rid of heat.
Lots of other materials would serve, I wanted something that is a good insulator, would not absorb water if any got inside the case, not going to fail due to abrasion, Kydex seems to fill the bill, and I had a stack of it.