If I lived in a place that I could keep my monitors outside, I'd dig the area 3' deep, lay down 2 or 3 layers of hardware cloth that extends 2' past the perimeter area, then install chain link all the way down to touch the hardware cloth, and use wire to bind the two together, then pour cement along the crease where the two meet. After that sets, I'd fill the area back in with the 3' of fill dirt/sand mixture.
This prevents any sort of digging out while still allowing for decent drainage. The chain link should go up at least 5' over ground, and then I'd go around the bottom inside of that with vertical boards to prevent climbing. the boards would have to be a sturdy and sealed plywood, you'd be amazed what monitors can climb.... Or better, you could use that solid-sheet plastic fencing stuff.
I'd make a large hide box, set a planted pool in there, and toss in a bunch of huge logs and rocks and bushes.
That allows for hiding spots, basking spots, and with the hardware cloth so far below ground, they could even build burrows without worry.
Hope this helps!
