I have the same tank. Do you think he could have gotten in through the slots at the bottom of the styrofoam? I've worried about that myself, but so far my BP hasn't gotten back there.
I would move him back into the tank part. You can try stuffing those slots with paper towel or newspaper--I've done that with a previous Exo Terra tank I had to keep crickets from getting behind the background. I wouldn't use glue or silicone to adhere the background to the glass unless you're not using the slots for wires (which it looks like you are) and don't need to get back there again.
Good luck! I don't know what will prove trickier to keep in an Exo Terra tank, snakes or crickets...
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