There's a neat trick to this. First I will say coco fiber works best but mixed dirt will do fine if it's dry.
I leave It about 4 days to cure but it varies and here's why. I use Motorcraft clear silicone rubber you can get this at auto supply stores or a ford dealership. First off the texture of this stuff when cured is very soft "earthy give" yet strong the second reason I chose it is because there is no guessing at all it has an additive that gives off a vinegar smell that won't quit until it is completely dry and safe for your spider.
I haven't seen anyone use this silicone before. I didn't know Ford had a chemical division or chemical products either interesting. I like the vinegar smell/curing, very helpful.
What are you putting in there?