I would not use Envirotex for a monitor cage. I think the cheapest way to seal the inside of a monitor cage is to use fiberglass resin. However, the solvents are very nasty and it's a pain to work with. If it were my cage I would go to Home Depot and buy sheets of FRP board, also known as fiberglass reinforced panel. Cut it to size and attach it to the cage walls and floor with Liquid Nails or a similar adhesive.
If you need more information on this do a search on both the current monitor forum and the archived monitor forum. Both are a wealth of information on both resin and FRP board. In fact the monitor forum is a great place to get advice on cage construction. At least it used to be.
Neither resin or FRP will make for the most attractive cage interior, unfortunately, but you still have the exterior of the cage to make it look nice. My two favorite of the readily available woods are birch and oak. Birch with a mahogany stain has a nice cherry look to it. Oak can be stained with some of the lighter browns or honey colored stains to show off the grain. Both birch and oak laminated plywoods are readily available at Home Depot and other places.
If you really want a stained wood interior with a durable clear coat you'll have to ask some questions and do some searches over on the monitor forum(s). Many of the more durable epoxies have a cloudy look to them when cured but I don't know which ones. I wish I could recommend Envirotex but I just don't have any experience with it and species that scratch a lot.
Sorry for our e-mail trouble. Let me know what further information you need.
Regarding your choice of monitors, I have never worked with the blue-tailed. I have worked with mangroves and peach-throats. From my limited experience with them I liked the peach-throats better because they seemed more arboreal and displayed better. I have no idea if that holds true for all specimens.