Hello, you had so many good ideas. but if you can bear one more idea..
I purchased 1 sheet of foam core. The sheet is 4feet by 8feet. This works for me for now. I do have to use pins to hold the squares. But when I can, I will purchase some off-white flannel to put on top of the foam core. I also plan to by a second sheet. Then the quilting wall will be 8feet by 8feet. The sheet of foam core was under $11 dollars. I am fortunate to have a company by the name of Allied Plastic Supply in my town. But you can do a search in a town near you.