I started by buying two 20" x 15" poster frames from target. I liked these ones because they were deep (1.5") which would be good for defusing light, and I just removed the clips that normally hold pictures inside of it. I initially thought I would use one enormous picture frame, but settled on these realizing I could use 2 and then it could fold for portability. 

http://www.target.com/p/umbra-loft-6-opening-photoclip-frame/-/A-14126361#prodSlot=_1_20