I would be concerned that a hatchling would try and push its way out of a 1/4" gap.

Have you thought of building a rack that fits the dimension of space you have?

How are you heating that drawer thing? I would just get a temp gun and check temps once a day rather than having a thermometer in each tub.

Drill about 10 holes to start and see how the humidity is in the tub (make sure you put in a water bowl with water in it). If the humidity is too high, make more holes. It really just depends on the climate you live in. There is no set number of holes.

If you can make it work, then by all means go for it. Just be aware that many people have problems with that kind of tub setup and usually ditch it very soon for a rack setup.