Thanks for this, Jeff! I'm planning to have two MERV-14 cubes going at an upcoming contra-dance. I'll refer curious people to this post.
I couldn't find 20x20x1 filters at my local hardware store; only 20x14x1 and 20x30x1. I don't have a good intuition for how effectiveness relates to filter size. How much less effective do you think a 14-inch tall "cube" would be than a proper cube?