Foldable A5 writing pad

print both halves, then pull a 2mm elastic rope through the holes as shown on the photos and make knots at both ends (creating some tension in the process, don't overdo it though). You can optionally glue in some magnets (watch the polarity) in the holes on the outer edge to make the pad snap shut decisively. This is not strictly necessary.

To unfold, open both sides to a flat surface while simultaneously pulling them apart. Once they're flat, allow the rubber bands to pull the halves together, and then push them together until they hold firmly. Reverse process to fold. The “fingers” are dovetailed and lock into each other quite rigidly and provide quite stable and firm writing surface (obviously, don't push it).

This pad works surprisingly well for writing on A4 sheets as well - just place the pad behind the  section you're writing on and hold them together with your non-writing hand.