Thanks so much for the kind words!
Designing levels is actually much easier than solving them ;) I just stamp the shapes on with addition in any way I like, it's up to the player to subtract them again. It's thanks to the design of the shapes and the fact that subtraction is commutative (order of operations doesn't matter) that naturally leads to multiple possible solutions.
I did try to create interesting levels, not just random ones. I've found the truly randomly generated Unlimited mode to be much more difficult than the hand-designed levels.