Go to the hardware store and buy yourself a couple of 4 x 8 sheets of pink foamboard, the thicker the better, a roll or two of duct tape, and a good sharp utility knife.
Make yourself a big box -- cut one sheet in half so that you have two 4 x 4 pieces. Cut the other sheet in half into two 4 x 4 pieces, then cut each of those lengthwise so you end up with four 2 x 4 pieces.
Tape the 4 sides together into a box on the 2 foot side so you have a box that's 2 feet tall and 4 x 4 feet. Then, tape one of the 4 x 4 sheets onto it for a bottom. Then with your utility knife, cut out a small hole on one end about the 3 inches in diameter, towards the top of one of the sides, and a corresponding hole at the bottom of the opposite side -- this will allow just a little air flow and oxygen without too much temperature swing.
Finally, put the whole apparatus on a stable surface in whichever room in your house has the least temperature swings, such as a basement. Put the incubator in it, centered as much as is possible, then put on the lid and use a couple of pieces of tape to hold it in place (you will need to take it on and off, so just tape loosely.
This will greatly reduce the temperature swings. I would recommend that you run the incubator in place in the box for a couple of days with a few dummy eggs inside (store eggs or even something like a few small potatoes or a couple of ziplock bags with about a half cup of water in each) just to get the thermostat settings right before you put hatching eggs in it.
