You should not be using substrate anyway. It harbors bacteria and encourages mold growth. It could also impact you cham if it accidentally are some of it. Most people use a bare bottom or just use papers towels.
I always use those hydro balls in my terrariums. And yes I do keep two of my chams in terrariums. Also if you look at Josh's Frogs, a forum sponsor, you will see they recommend the use of hydro ball.
You will need a large terrarium for that girl as she will grow pretty large. I would use at least a 24, 24, 36. If you are just worried about eggs, you can put a laying bin in a screen cage if you want. Where are you located? That may make a difference in what type of enclosure you want.