I think you could build them at this size with minimal sagging. To minimize sagging you would want the lower most cage on a perfect flat and supported surface (placing the bottom cage on a 2" thick sheet of styrofoam works well to achieve this) and also including upper and lower lips inside of the cage for support. I would probably also include a few strips of plywood hidden behind the upper lip for extra support.

All of the bartop epoxy resins are very similar. If you're building multiple cages at once I would order Kleer Koat in bulk as it's still the cheapest price I have found. You should compare the price with shipping to Envirotex Lite that you can probably find locally. Last time I checked with shipping costs Kleer Koat was still cheaper if ordering at least two gallons.

I like the bartop epoxies but they go on so thick that they end up being expensive. You can pour them thinner but this is a lot more difficult across large surfaces. I would not recommend this if it's your first time. You need to warm up the epoxy to get it to run thin which means it sets up quicker. Not easy to do. You could possibly get away with this for the walls but not the floor of the cage.

West Systems epoxy with one of their specific hardeners cures stronger that bartop epoxy and looks as good or better. On paper it looks very expensive but since you can brush or roll it on you can use a lot less, thereby saving money.

As far as hollow core door blanks, I think they offer some advantages. They tend to be very close to perfectly square and flat, unlike plywood and obviously sag less than plywood given their modified torsion box construction. If you look in your paper or at building supply stores you can often get used ones for cheap or even free. And even better, the older doors tend to be of much better quality.

I have only weighed on HC door, a cheap on at that, a cage built from it would have weighed about the same as one built from 1/4" PVC and considerably less than one built from 1/2" PVC or HDPE.

The big advantage for you is that they are darn near the size you already want. In your case you would just adjust the size of your cages to 80" long. I don't believe it would be worth it to cut 8" off just to get a 72" long cage.

The primary concern I have with HC doors might be a strength disadvantage of the thin plywood veneer on the cage floor. If you have a lot of substrate I would worry about flexing inbetween the interior grids. One obvious argument against this is that wooden kayaks and canoes are made from similar plywood and they withstand much greater pressures when in the water and carrying a full load. Of course these boats also have a layer of fiberglass cloth which you could add in the future if you needed it.

I have more thoughts on HC door blanks but I'll hold off on that for now unless you pursue that route.

Home Depot does not carry the true 5x5 Baltic Birch. You'll likely need to get it at a lumber yard that caters to the cabinet industry. Or maybe even buy it direct from a cabinet shop and have them cut it for you. Even if you found it at Home Depot I would not have them cut it. Due to corporate policy they are not allowed to clamp stop blocks to their panel saws which means results are not always so good. I have had them cut wood but it is always hit or miss.
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6.10 Gonyosoma oxycephalum (Javan, mixed colors)

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