I was at Home Depot the other day and they were selling Grip Lock clear containers for 99 cents. These could be perfect as an egg tub because of the size and it is clear so you can see right through the entire thing. The lid snaps shut around the handle area but it is very loose fitting compared to the tupperware containers so I think it would be good for air exchange.
Also, check out the Accurite indoor/outdoor thermometer/hygrometer with probe. They sell them at Wal-mart for around 12 bucks. They have a more expensive version - around $40 - that comes with a remote sensor... that is, you can put the sensor with probe inside the egg tub and the display unit can be on your bedside table. I've never really used that model because I already got the $12 model.