Columbian Tegus typically get 2.5~3' long and are quite active. He'll definately need more than a 90 gal... Also, Columbian Tegus are difficult (but possible) to tame. Keeping him in a small enclosure will make it much more difficult...
But anyway...
I've made several 'Vivariums' or 'Paludariums' for amphibians in the last few years. I carve them out of styrofoam and then paint that styrofoam with Drylok. Drylok 'seals' the styrofoam as well gives is a 'rock like' finish. I am very very happy with the results of using this combination.
I've also used brown silicone smeared on carved styrofoam to create a 'bark like' finish.
Just to note, Drylok can be tinted with regular latex paint pigments/dyes. Corporate big box stores (like Home Depot or Lowes) may not be willing to add them as doing this voids the manufacturers waranty, but smaller hardware stores (like Ace, Aco or non chain) will usually do so without much hesitation. Explain to them what you are using it on and acknowledge that the waranty does not cover this application anyway.
I even talked the Ace clerk into putting several colors of pigment/dye into small containers (that I bought there) so I could take them home and mix small batches of different colors myself. This really opened up creative options at natural looking environments.
...and of course, no one is happy without pictures
this is a 30 gal tank for Rubber Boas. It's a dry tank but uses some of the concepts described above. I carved the Styrofoam background then painted the whole thing with dark grey Drylok. I smeared (clear) silicone on the "tree" then pressed EcoEarth (pulverized coconut husk) into it. For the light tan sections I smeared (clear) silicone and pressed pulverized walnut shells (sand would have worked also) into it.
This is a 48" x 18" @ 13" tall 50 gal aquarium divided in half (eash side photoed seperately). Again, the structure is carved Styrofoam all painted with tinted Drylok. The 'tree' in the back corner is GE's darker brown silicone, the 'log' in the front is GE's almond colored silicone.
Drylok covered styrofoam works great for amphibians, snakes and small lizards. But I would not use it for larger reptiles (like your Tegu) as the claws will quickly and easily destroy structures. You could do the same thing but replacing the styrofoam with wood (I considered doing this for my Arg Tegu's enclosure), but not only would it be a lot more work but it would weigh a ton (weight is the reason I chose not to do this).