I would use plywood over acrylic - acrylic conducts heat well and the heat coming off of the top of the RHP will definitely be an issue. Also not sure if the lid would hold the weight, it may need a framework on top to hold the screws securely.

I would use two UTH's instead, set one for a 90*F hot spot and the other in the low 80's. Put a hide over each.

Would it be feasible to run a small space heater in just the room they are in?

Another thought is that I have the small chest freezer in the snake room. The exhaust from that helps keep the room toasty in the winter if I close the door, even if the rest of the house is cold.