To give an outline of my thoughts, I'd like to make the cage dimensions somewhere along the lines of 60" x 24" x 36" or, preferably, 72" x 24" x 36" (length by width by height)

1.) Is there a particular plywood that would stand out as more efficient for this type of enclosure? Particularly one that wouldn't sag at lengths of six feet.

Medium density overlay (MDO) is supposedly a bit more rigid than the standard plywood but more importantly has a resin impregnated veneer that is exterior rated and has some moisture resistance. I would still add more moisture resistance but MDO is a good start. It used to be very inexpensive but I understand its price has increased more than other plywoods. Not sure why.

Most plywoods come in 4x8 sheets, meaning a lot of waste with the sizes you mentioned. You might consider tweaking your dimensions slightly. Or you could go with true Baltic Birch plywood which comes in 5x5 sheets and maybe have a base size of 60 x 30.

2.) Would it be a good idea to line the cage with FRP? If so, how would I go about laminating the FRP to the plywood? Is there a better alternative for waterproofing?

I'm not a fan of FRP unless it is needed for abrasion resistance -- like a large Varanid lizard scratching on it. It does work very well for that. And again, in the sizes you mentioned you'd be wasting a lot of FRP since it comes in 4x8 sheets.

There are various options for sealing plywood, including vinyl film, contact paper (shelf liner) or other sheet or roll plastics.

Then there are the paint on liners, which can be epoxy resin, polyester resin, and both solvent based and waterborne epoxy paints. Even cheap Drylok seals plywood, although it leaves a rough and hard to clean texture. Some of the non-textured truck bed liners have also been used.

3.) I plan on mounting the heat panels directly to the FRP interior; is it possible that the heat could cause the FRP to separate from the plywood?

If you use regular FRP adhesive, then no. Besides RHP's have a radiant barrier so the back of them actually does not get that hot.

You might want to pick Nate's brain about an idea he has brewing for large cages. It might sound expensive but once you price plywood plus a sealer you might be pretty close to what he has in mind.

He and I have been discussing this offline so feel free to email me as well.
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Currently keeping:

6.10 Gonyosoma oxycephalum (Javan, mixed colors)

1.1 Philodryas baroni

1.1 Lampropeltis triangulum multistriata

1.0 Rhodesian Ridgeback

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