Open this document, and scroll way down to Egging, Gravidity. Or Gravidity, Egging. There you find instructions for building nest boxes. Egging females can be fussy, and you want to maximize her shot at laying all of them by making a good one:
Gravid igs sometimes reject even the most perfect nest boxes. That would lead to an expensive egg-bound spay that can only be done by a vet experienced in reptile spays.