I have finally finished building a mini incubator for my single pair of BP's. This is made from a styrofoam cooler, 4 inch heat tape, some plexiglas, and hot glue, water bottles and duct tape.
Pricing: Cooler: $0-20 All but free for me, came with my recent Layne's labs rodent order
Heat tape: $10 (20-25 with shipping)
I bought 3 feet of 4 inch heat tape from reptile basics, as well as a clip/insulator/plug set. The heat tape was ready to plug in from the start (thanks Rich!)
Plexiglass: $0-10 I had this scrap lying in the garage, but it is relatively cheap to buy at Home Depot
Note: Plexiglass is 100% optional!
Water bottles+ hot glue+ duct tape: $0
You will probably have these things around the house.
Now onto the fun part, the build!
Step 1: Decide where you want the hole for cords. I made mine in a corner, and just pulled enough styrofoam off so that it would close easily with cords
Step 2: Run the heat tape across the cooler along the sides and bottom, then secure it with duct tape. (aluminum tape would be better, but duct works fine)
Make sure to leave the heat tape cord on the same side as the hole you made.
Step 3: place as many full water bottles as you can across the bottom of the cooler, on top of the heat tape. These bottles help evenly distribute the heat, and keep the temperature much more stable, especially in case of emergency (power outage, etc)
Step 4: Optional
Using a kitchen knife, cut a hole large enough for your plexiglas. Be sure to cut with the knife slanted towards the center, so there is no way the hole will be too big. Secure and seal the edges with hot glue.
Now your incubator is ready to be plugge in to a thermostat. I recommend a proportional thermostat such as a herpstat (my personal preference)
Hope this helps!