Welcome to the world of DIY enclosures. Personally I think that it is the most satisfying part of owning reptiles.

Plywood is a great option to start with. I would get to a big box construction store and have them cut down a piece to your exact dimensions. I usually go with 3/4 inch pre sanded boards that come in 8x4 slabs.

You will get a lot of differing opinions on how large to build your first enclosure. I would suggest going to a monitor specific forum for sizing.

That being said, I've had the best luck with essentially just creating a wooden box that has one side panel open, where you can install either sliding glass or a hinged door. For heat and UVB I utilize two hanging wire cage lamps with one in the center with a CHE and one housing a low wattage bulb for a basking spot. I also have a t5 ho strip uvb light with reptisun tube light. All of these heating elements are INSIDE the enclosure, hanging from metal chains attached with hook and eye loops drilled into the ceiling. This allows for retention of both heat and humidity. Savvy monitors are not desert dwelling animals like some believe, and they need sufficient humidity.

As for sealing the cage, go with Drylok over epoxy. It is tough as nails, completely water tight and easy to work with. Dries fast with very little smell. It also has a gritty texture that helps with monitor claws. I use this in my sulcata setup and it works great. It can even be made to seal homemade fish tanks.

I know this isn't a comprehensive construction plan but it's something to start thinking about.