I decieded to flip over a couple of canvasses that I had one hand and paint the BACKS of them black.
I then put a block into the recess in the middle and put on a few dabs of glue (glue gun glue is fairly easy to remove in case I change my mind)
The result is a 3D "frame" which sets off the pieces, allows me (and visitors!) to touch them, and cost me nothing since I had the canvasses and paint on hand!
I LOVE how they turned out!