It depends on how the top of the aquarium tank is designed, you could try finding that model online or similar tank and try to see if they sell lids for it separately. Although the lids being sold for it are most likely going to be all covered since it's for fish. You'll probably just have to build one with a frame made out of wood or plastic lined with aluminum mesh found at hardware stores to make it either fit on top snug or slide in. However you want to do it, and the easiest way it's going to work. It might look kind of like a unfinished project, but if you want to save money then that's how I would do it. Or go on craigslist and look for a deal.