Depends on how long it going to take you to build the enclosure and the size of your new croc.
Croc monitors like to be above ground too, they spend maybe 20%-30% of the time above ground, they also like to have a hiding place on the grounds too (covered spots, like those 2 areas under the lights in the pic of the "portable". You need to leave some larger walking area's, thats why the whole bottom area other then the water tub is empty. Honostly that water tub is too big, but that monitor like to eat the live fish i also breed and the larger the tub the longer the fish last. You can make shelves, they are better then trees as they provide much more space, you can also put in some trees to if they are lite, id suggest put in some dowels or thinner limbs(about the size of empty paper towel centers) thinkness the whole length of the cage or partial and screw them in tight not nailed, it will add some steangth too while meeting some of the aborial needs, also make shelves, the problem i had when i put heavy trees etc. in the portables im making is that they get heavy fast and they are made from only 1/2 inch plywood and glue nailed (it will warp if they get to heavy, i tried it already). As far as humidity goes just dont put in large vents, put very small ones in. you may not need a vent much ventilation at all, depends on how tight you built it, if you can see a little condensation on the glass, usualy start at the bottom your usualy fine. Ill put in some dowels in the corners etc. if i have a chance today, the dowels or even some 2x2's or 2x3's will work fine to but consider everything you add will make it less-portable.
It should take you 1 day to build it, the reason i made a 4x8 is because i dont like to cut (if i could have made it larger i would have went deeper, not taller, thats why all my enclosures are usualy 4x8, 8x8, 8x12 etc.. Consider if you dont mind cutting, the differnce between a 4x8 and a 6x8 is massive, but its wood wastful. Ideal size is 4 foot tall, 8 feet deep and 8 foot wide, you wont have to ever build another cage most likely, i dont know if you will be able to move it though, i will know soon. You can put a more permanant tub in and plumbing, but i just wet-vac it out manualy. Also use more pete moss in your sustrait as the dirt is heavy.
Lets say your new croc is a 4 footer, most are about that leangh because 99.999999999999999 are wild caught, dont listen to these people who say CB croc's are common, there not, they just dont know better, CB tree monitors are the "new" scam. You can put a 4 footer in a 100 gal aquarium for a few days, just cover the sides up so he cant see though it and a mesh top or whatever you have, if a mesh top cover it with a thin sheet or somthing over it, monitors, especialy croc's use there tounge to feel there way around as well, the monitor will probably rub its nose on the wire mesh top if his tounge can fit through and mess himself up, the sheet will shop its tounge from going through the mesh and he wont think he can get through. You wouldnt keep him in it for long though.
