Foamboard is a great idea...but...
Michael's sells Elmer's (yup, the glue guys) foamboard. I strongly doubt that it is acid free in any way, but that's just an opinion. Most that is sold as acid free is not entirely safe for archival use. The foam itself actually produces gasses that can damage artwork (major orangish discoloration to photos and prints, not sure about fabrics).
Bainbridge makes one (Artcare Archival) that protects against this "outgassing" and that is what I get from a local art supply store for my prints. A 32x40 sheet sells for 6.25 and the 40x60 sheet is 11.75. If you have to get it on line, any savings will be lost in the fact that you'll have to pay oversized shipping rates.
Thanks for the acid testing tip, Moonpi. I'll have to remember that!