Summary

This adorable two-piece laser-cut design is an exercise in minimalism. How basic can you get while providing some protection?
To allow use of a GPIO ribbon cable, the top is cantilevered — it attaches only at one side, with a dummy standoff to provide support.
Includes GPIO pin labels. Side nubbins prevent accidental SD card ejection.

Instructions

Insert 4 screws into corner holes on bottom piece (screw heads will act as feet). Add a nut to each, tighten.
Lower Pi board onto screws, SD card slot facing the edge with the slight bumps.
Add nuts to the 2 screws flanking the GPIO header. Tighten, then cut the screws flush with the face of the nuts.
Using a 1/4" standoff as a visual reference, cut the other 2 screws so their protruding length is just slightly less than half the length of the standoff.
Install 2 of the 1/4" standoffs on these screws. Board should now be secure at all 4 corners.
Insert 3 screws in the top piece. Using the remaining standoff as a reference again, cut all three screws so their length equals the acrylic thickness + slightly less than 1/2 the standoff length.
Install the remaining standoff with a screw in the hole near the USB cutout (near the label for GPIO pin 5). There's no mounting hole here, this piece is just the cantilever support.
Position the top piece over the board and install the remaining 2 screws into the standoffs.

Minimal Raspberry Pi Model A+ Case