What kind of cage are you building? I have seen them with pvc drain covers from Home Depot which work well and if apearance is not an issue. You can also upgrade to brass or chrome drain covers for a cleaner look. Of course these are for round vents. If you look at woodwork supply houses you can get oak or maple vent covers in various sizes, and also small plastic vents used in cabinets to provide computer and a/v components with circulation. Any of these can work. One recommendation (depending on the size and sofistication of your cage (is it for monitors or tegus?) put a couple of small (3-4inch vents on ths sides toward the bottom, and then put a closable louvered vent near the top. The heat rising and escaping through the top vent will draw fresh air thru the lower ones, and by closing the lovers you can control heat retention when necessary. Hope this helps. I also prefer building my own cages. Enjoy....map