How to store a credit-card-sized FPGA+CPU board?
Considering Adapteva themselves offer a 30$ case... go for that; it looks well-made.

Other than that: seriously, the PCB has four screw holes:



So get any enclosure you want, and put screw standoffs (the little things between your PC motherboard and PC case; everyone who's been building PCs should have a couple dozen laying around) and screw them into that case (drill holes in the right positions first).

If you want to leave the board out in the open, do so, by using just about any board (wood, PCB, steel, …) instead of an enclosure.