You have a nice project going on here
I wish I had read this a while ago. I hate Hate HATE seeing people BUY styrofoam sheets. This stuff is available for free by the carload at any furniture store. I've worked in the business for over 5 years and have thrown away hundreds of cubic yards of the stuff in all shapes and sizes. The kind of stuff that would work PERFECTLY for DIY backgrounds. And I can get sizes and densities you could only dream of getting at a hardware store.