I recently built one of these for a tarantula. I used Great Stuff to foam-in around the pieces of wood and bark I used, and to add mass where I wanted it. The Great Stuff is SO easy to carve! I did it all with a box cutter and a razor blade. I coated the carved foam with silicone and then coco coir.
I would run screaming from tile grout. That stuff has lye in it. If you've ever had your hands in it for any length of time, you'll find that you have to wash them frequently or your skin starts coming off. That would seriously endanger any animals in the viv and probably kill plants as well with the imbalanced ph.
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