For that size cage I'd do something entirely different. A melamine floor won't hold up long term to the urates/deficates produced by a boa. I like to use something more durable for the floors.
So if it were me I'd build the melamine cages *without* a floor. At least not a melamine one.
Then staple/silicone a piece of expanded PVC sheet directly to the bottom rim of the cage.
The cage then can be set directly on top of the flexwatt or whatever you use. The cage will be lighter and will have a floor that is very simple to replace.
I'd also line the first few inches of the cage walls with packing tape or contact paper.
But ultimately I agree with Mark. Under tank heating is really not a great idea for heavy bodied snakes unless the cage is already in a relatively warm room.
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1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)