My goal for this project is to create a safe indoor or calm weather outdoor quad similar to Rolf's "annoying mini UFO". I started with an 18.25" square piece of EPP foam that was about 2" thick. Knowing that the motor-to-motor shaft cross distance was 230mm and the prop size was 5", I marked up the foam for openings that allowed a 1/4" gap around the props.

So far, the foam block is just rough cut using a hobby saw. The EPP foam is fun to work with and I can bounce the cut block on a hard floor without damage.