[chairman]

It sounds like you want about 4 or 5 inches of dirt and 1 or 2 inches of mulch.

For the mulch, instead of oak, I would use coconut husk mulch. I have used ReptiChip in the past, I'd recommend looking at it as an option.

You don't want to use compost. It tends to not work well in indoor enclosures. Just use regular top soil if you want dirt.

I would use all purpose sand instead of play sand. All purpose sand is coarse and its grains vary greatly in size. It works very well to hold the structure for burrows (yes, beardies dig burrows when given enough dirt) and it serves as a service for microfauna just like play sand.

I would strongly consider using a 70 lb bag of coarse sand, two 40 lb bags of top soil, and fill out the rest with the coco coir bricks. It may be a little dirt heavy for spineless opuntia cactus but should be good for mulberry.