Here is another thought. You can always place a smaller tub enclosure inside an un-divided T8, use the full sized RHP, and just place the tub where it gets an appropriate gradient. (Obviously some experimenting needed)
For ventilation I'd leave the doors open/removed on the PVC while using the inner tub, but this will give you time to see how ready of an eater your snake is and let you get a feel for if he would be more or less comfortable in the full cage. (Some are just very shy and this will be closer to what it is used to to gradually acclimate the snake to a new environment) A 28qt sized tub would be a good middle ground so he gets a bigger space, but not huge. And would save you from possibly needing 2 RHPs or buying different heat stuff for a stand alone tub.
This may only be needed for a few months and would be cheap. The RHP would do a good job heating it and save you from having to spend extra on heat equipment.
I've done this with a new BP before to good success.
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