I am glad your thinking of investing so much for your monitor. I hope to not stop you, but instead give you the reality of what your facing.
The size of the room is fine, but the problem is, its a room. Which may mean its it your house. If I am wrong and its an out building, then ignore the following parapraph.
In order to keep the room suitable for an adult water monitor, it will destroy your room. It will rot the floors, the walls and wrap the entire structure. Hi humidity does that indoors. Normally the design of houses is with low humidity in mind. So keep in mind, your room most likely will be destroyed, possibly much of your house.
Of course you could line all the interior walls with FRP panels or something equal.
You could also keep the room too dry and this will have effects on the health of your monitor. In reality if managed your monitor may only live ten years instead of 25, which may be alright with you. As in, thats your choice.
Also, you mentioned its for a water monitor, wheres the water? Yes, keeping large amounts of water in a house is also ruins the house. Maybe a better choice is to keep a species that does not need swimming water. You can keep a water monitor without water, but then whats the sense of that?
About your substrate setup. I think you fail to realize the abilities of larger monitors. I did too. A tiny story, I decided to make a leaflitter pile for my lacies, I went and gathered leaflitter, I raked for hours. I collected leaves and stacked them in a pile about 10ft by 10ft, and over two feet high. The point of the story is, the male lacie went over and stuck his head in the leaves, then move the whole pile in a few minutes. I soon realized the similarity of their feet and a rake. This goes for dirt, the problem with making piles on one side of the cage or making a small box is, they move all the dirt in minutes. They move it out of where it was and over to where it wasn't. So your dirt box is not the best idea. It is an idea, just not very workable.
And yes, there was a fella that had an out building and converted a room for water montiors, He made a big pond and deep dirt and yes his monitors did well and actually produced babies at least once. But it was not long after he got rid of his waters. I really don't know why, but I have an idea that the cost and the work was too much to bare for long periods. You see, monitors live for long periods if taken care of. I have a male lacie that can be tracked back almost 22 years and hes going strong.
Which goes back to the original point, how will your room hold up to high humidity and water and moist dirt for long periods of time, this is a prime consideration.
ALso, how about service, for instance, I have a monitor building, not only are the cages water resistant, but the whole room is as well. And the rows of cages all have wheelbarrow access. How do you get all the stuff in, and better yet out? Moving stuff in is kinda fun, out is just nasty work.
You do understand that natural materials contain natural stuff(bugs) What we found was, making a large cage with natural materials also creates the oppertunity for invasive and feral creatures(I just relearned the difference) And in hords. For me, the biggest setbacks have been invasive creatures. From horrid blooms of the bugs that come in crickets, to black widows to kissing bugs(the worse)
AT one time, I would kill and this is an under estimate, millions of black widows a day. Of course I got lucky and some other kind of spider ate the widows and they do not make massive amounts of babies, and they don't bother me. Also, monitors did not eat black widows, but do eat the other type of spider. So now black widows are rare. The kissing bugs are the worse, they not only attack the monitors, suck their blood, but they attack me. The black widows did not attack me, thank god.
The above paragraphs brings about a reality. If this was in my house, a couple of things would have occurred. My wife and family would have first killed me, then burned the house down, or visa versa. Or I would have killed off the monitors, bugs, burned the house down and done in my family just to clean the slate.
I wish you luck, and you do understand, if you have a male, they get really really big. Which means, really really big piles come out the other end. Cheers