I'm not sure how sturdy the lid is on this tub, but if you were going to mount an RHP, I think you'd want to mount plywood or something similar on the inside since the RHP would be in direct contact. The surface of the RHP can probably get to 160°F which could warp the plastic lid.
Another thing to consider is your thermal gradient. The 120W RBI RHP is 32" long and the tub is 52" - not sure how good of a gradient you would get in the tub.
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