Foam from Lowes or Home Depo is cheap. They have 20 different types. You can use two sided tape to put it on the glass. An remove it when you want to. Some people will glue it on the sides an back this makes removal messy. I use R-max because I have it left over from a job an it's not cheap, so I didn't toss it. Styrofoam is cheap. Some types here:

https://www.homedepot.com/b/Building...n/N-5yc1vZbaxx

For the top I use hard cardboard wrapped in foil. It keeps the heat in.

Good luck!