We finished building a box to hold a couple cats this weekend. Since Bean discovered the feline allergy, we've had the cats all outside all the time. Now it's getting cold, so we built a box out of 1/4" ply, weatherstripped it, caulked it, build a catdoor out of an old harddrive magnet, a couple nails, some silicone caulk and some greenhouse plastic.
We hinged the top, which was the only thing we actually purchased for the project. Total cost: $2 and change, plus getting rid of some scraps and an old half-used tube of silicone caulk.
Making stuff is cool. Having a shop and the tools I need is awesome. So blessed.