I constructed one of mine out of a piece of plywood, a 2x4 and some leftover carpet that I am using for the bottom of one of the enclosures. If you have tools and are handy, it took all of about 10 minutes. Two short segments of 2x4 for sides, plywood on top, and carpet stapled over it to form a 3-sided box. Cut a hole in the carpet for entry, and presto, a hide.
If you're not feeling crafty, go to any discount home goods store and find a wicker or similar basket and flip it over. As long as the handles don't protrude beyond the top of the basket, it can lay flat in the enclosure and the handle openings are where they can get in. I cover mine with a hand towel to keep the light out. You can kind-of see it on the left in the picture.