Building your own can be very cost effective and is probably the best way to go. Finding affordable enclosures outside of the typical size range is very luck based (craigslist ads, facebook BST pages, etc). Its really hard to mess up assuming you have the proper tools. I'd recommend a staple gun, drill / screw gun, level, and carpenter square, screen, 1"X 2" lumber, hinges, latches, measuring tape, and metal corner brackets from a hardware store. The supplies should be fairly cheap compared to a professionally made enclosures.
My chameleon enclosure was 20-30 dollars to build and it is 28" X 24" X 48". I probably could have doubled the size for $10 more, but I do not have the space for that. It took roughly an hour to build and it came out very nicely, especially for being the first chameleon enclosure I built.
If you end up building your own, consider buying 10% off Lowe's coupons from Ebay. They are cheap and even obtainable yourself. Home Depot readily accepts them, and they can save you a good bit of money, especially if you are buying live plants for the enclosure.
Other than what was stated above, buying a pre-made enclosure from a company such as Dragon Strand is probably your best bet. Just take a measuring tape and create a template on your floor of how large the pre-made enclosure is, and you'll have a good idea of how much space the enclosure will take up.
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