Very Inexpensive Window Insulation Insert with Foam Board

Credit to https://www.youtube.com/watch?v=XqIWzNi6y8w&t=378s

She has a very frugal method to make insulating window inserts with foam board for a standard 35"x58" window. It might also work with heavy-duty corregated cardboard.

I really like this option over bubble-wrap on the windows because I can use a clear tablecloth liner and be able to see out of the windows.

Most of what I found for building these inserts used screen kits or furring strips. The screen kits look relatively easy to cut and build, but they are ~$20/window. Furring strips are only $3.20/window, but they are much thicker than I'd like and wouldn't fit behind the blinds.

Foam board is $2/window. It's the lightest, easiest to build, and thin enough to fit behind the blinds. The downside is that it can be damaged/bent easily.

Does anyone have an idea for a wood, metal, or plastic option for the frame?