I want to preface my reply with the following: These are my opinions based on my own research and experience. Others will have differing opinions for their own valid reasons. Much of this is subjective, but you asked, so here goes.....
I have a 6'Long x 3 1/2'Wide x 3' Tall space availible, i was wondering would this be big enough for a relatively inexpensive, not as much as the custom build ones that you can buy,?
This sounds like sufficient space, if you intend to build your cage to fill the whole space. Mine is 71/2 x 2 3/4 x 3 1/2 high, and it currently houses 2 juveniles. I designed the cage with elevated basking shelves and an elevated pool, thereby giving more square feet of space by creating hides beneath each structure.
The general wisdom says you don't need heighth, because tegus are terestrial, but I have found that ours climb all the time. The are often up on the plant ledge, or climbing on the waterfall pools up near the top of the cage.
Which is better to use and cheaper, Melamine or Plywood polyurathene?
Neither is cheap (at least in CA where I live.) I used birch plywood for mine, but mostly for esthetic reasons. The tegus live in our family room, so I wanted a furniture-quality finish. Melamine just looks too sterile and unnatural to me. (My opinion.)
"Cheap" is a recuring theme in your question. Keeping tegus is not cheap. If you have to compromise too much, it will be the animals that suffer. Build the best you can afford, and build it to last. That will be the cheapest in the long run. Anticipate a 15 year life span, and the up-front cost won't seem so high. New housing every few years is certainly not cheap.
Could i use screen or is glass my only option,plus what are my other choices?
You do not want screen. The objective is to retain heat and humidity. Glass is best. Plexiglass or Acrylic will work but will scratch and discolor. I used 1/4 inch tempered glass on roller tracks. I would never try anything less. This has been ideal. I have 3 young girls who love to get their fingers all over it, and 2 young tegus who love to get their nails all over it. Glass is secure, it holds up without scratching, and cleans easiest.
Can i use what i have for the front also for the top for ventilation?
My ONLY ventilation is 1 (thats right ONE) 3 inch round vent on the back near the top of the cage. The rest of the ventilation comes through around the doors, and by opening them everday. The vent is near the top, the heat rises and escapes slowly, and draws fresh air in around the doors. It works perfectly, and maintains the heat and moisture idealy.
How can i build the locking/opening mechanism?
Try sliding doors. they're easy, don't require ugly hinges, don't require clearance in front of the cage, and can be securely locked with a cheap display case lock. (you see them all the time on pet store cages.)
Does anyone know how to build a custom waterfall, or can i just build a pool in, (this can be directed to "MAP"?
Definitely the hardest part. The waterfall and pool took the most time planning and building - but worth it!
This post would be tooooo long (as if its not, already) if I tried to give you all the instructions for the pool. I discussed it (and the shelves) at some length here, and also in the Cage & Habitat forum. Try searching in the archives for more detail. By the way, if you are looking for "cheap" a waterfall and pool is NOT the way to go. There are much simpler and cheaper means of providing water. But is sure adds alot of atmosphere, and the necessary humidity.
Whats your input on a bottom?
Mine is plywood all the way around (except for the glass doors.) For durability, and ease of maintenance I sealed the entire lower portion (from the shelves/pool down) with an acrylic marine resin. What I ended up with is a rock hard, water repellant surface. Perfect for high humidity, and lots of scratching and burrowing. Then once a month, I just vacuum it out, and scrub it down, and replace the bedding. Unless its well sealed, any melamine or plywood is going to take a beating, take on moisture, and begin to disintegrate. But long before it does, it will become a breeding ground for bacteria and infection.
I know this is long. Providing a good habitat is something of a passion for me. Hope this helps. Feel free to email me if I can do more.
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