Tonight I hacked together a quick enclosure for my 3D printer. When air currents cool your parts unevenly, you can have thermal contraction occur faster in one area than another. This leads to cracking, and makes it difficult to print in materials with high thermal expansion like ABS or Nylon. An enclosure helps to keep cold air out and hot air in, making the thermal environment a bit more stable.
The enclosure is super simple: A few strands of steel wire are twisted together to form a semi-pyramid shape. Over this, I used double-sided tape to attach mylar on the back and top sides, and 4mil plastic sheeting to the front. It's hacky and a bit ugly, but it seems to be stable enough. Time will tell if it actually works!