Thanks everyone!
I'm still in shock he showed up! He must have been really dehydrated because while still too skinny, he's looking a lot plumper since last night. We had kept a water dish on the floor the whole time he was gone, we just picked it up a few days ago. I'm wondering if not having the water dish prompted him to come out in the open.

Anyhoo, all set up, followed the tub set up sticky above.
No luck finding a thermostat locally. No private garden centers nearby, just big box stores.

I took the hydrofarm off the RHP in the big cage to use on the tub and am just waiting for the temps to stabilize before I put Monty in there.

I ordered another hydrofarm from Amazon for the big cage. I have prime membership so it will be here Wed. I figure there's no burn danger with the RHP, I'll be home most of the time the next two days so I can keep a close eye on the temp in the big cage plus it's big enough for him to temp regulate easily.