My first incubator was a mini wine fridge. Air circulation wasn't an issue for two reasons: one, the cord for the UTH that I used to heat the thing was out the door, causing a small gap and two, I checked on the eggs every week. The eggs themselves were in air-tight containers which I opened that once a week to make sure they were okay.

My current incubators are a pair of large styrofoam boxes that my clinic got shipments of vaccines in. I used dowel rods to create something of a 'platform' about 2 inches above the bottom, which is where I place a 20gallon UTH attached to a thermostat. The egg bins get stacked on each other in there, with the probe for the digital thermometer inside one of the egg bins. I can fit up to 4 clutches per styrofoam box.