I use an under mount mat made for aquariums. If she is happy with the box she will spend a lot of time in there digging.
The dirt/sand mix will be damp so she will be subject to getting to cold if the box isnt warm.
Iguanas have been known to hold sperm for undocumented times and could possible lay fertilized eggs despite no male being present. For the sake of caution , freeze the eggs at least 24 hours.