TailsWithScales

Best rule of thumb is you want at least 3 foot of cage space for every one foot of monitor. That’s inside open cage space. From the top of the enclosure to the top of the substrate. A hatchling /juvenile can be housed in a 4’ long x 4’ deep x 4’ tall enclosure but that will not last long with proper care. That will last you about ............. 6 months if you're lucky. I keep my 3 foot Nile in a 6' long x 6' deep x 7' tall and he uses EVERY bit of that cage. Climbs, digs, all that. So be mindful that this is an animal that will get between 4-6 feet long!
There are cages you can buy that are that large but they're more then $350 usually. Large enclosures are timely to build and require a lot of materials. It will need to be sealed to prevent water leaking or rot from humidity and to avoid mold getting into the cracks, so forth and so on.

Also these guys have A LOT of requirements. High heat, high humidity, a lot of food, a lot of time and patience. This is something to be mindful of also.

I can provide cage designs and building instructions with some time to create them but you could probably just find them online to but I'm not sure. Do you know someone who is a contractor or carpenter??

Also be sure to check your state /city laws BEFORE you buy. Just to be on the safe side.