Hi Mike,
I incubate all of my herp eggs in a closet. I use an oil filled radiator controlled by a basic Ranco on/off thermostat and keep the egg boxes on wire shelving. It can be set up in an hour for around $100.00, does not need to be insulated, is cheap to run, and most importantly it keeps a large volume of air stable.
If you are incubating one species, then circulate the air with a fan to prevent temp stratification and fill the whole closet with ball python eggs; otherwise allow stratification to occur and incubate different species on different shelves, ex. - bps on top at 88, monitors in the middle at 85, and colubrids down low at 80.
I messed around with hovibators, fridges and plywood boxes for years before finally setting up shop in a small closet. I slapped myself in the forehead and shouted "duh!" when I realied how cheap, simple and effective the closet incubator is.
I cannot take credit for the design though as I ripped it off from Dave Barker and Frank Retes. Good luck.
Will