If you can maintain the temperature above 80* in the box then your method will probably work. I incubate my eggs in a cooler I got from Walmart with a T-Rex heat pad inside hooked up to a thermostat. You can maintain the humidity by keeping them in a moist but not wet substrate like vermiculit, peat moss, or perilite. If the eggs are too wet they usually swell and the shell looks almost translusent. They can also start to row mold. If they get sunken in they are either too dry or no good. Infertile eggs will usually get sunken in and mold or turn colors. If it's your girl's first clutch it is not uncommon for the eggs to be infertile. To check for sure you can candle them by holding them up to a flashlight. If you see pink veins (capilaries) then they are fertile. If they are just yellow they are most likely infertile. I hope this helps and good luck!
-----
*****
Emily
3.5.3 leopard geckos (Nelson, Zander, Zoe, Baby, Houdini, Munch, Gizmo, Zig-Zag, Beau, Daisy & Gus)
1.1 ferrets (Oliver and Delilah)
1. budgie (Max)
0.0.1 Ring neck Parakeet (Kiwi)
1.1 German Shepards (Jake and Abigail)
The Spotted Gecko
**Updated 11-09-04**