You can also check at your local pet shop or online reptile supply sites and see if they have any cage clips that would work, or you can try using luggage straps from Wal-mart or Target in the luggage section. They extend out to almost 6 feet long, or you could try some bungee cords. I use luggage straps on my tubs because I think they have a cleaner look...
After reading your post again I realized you aren't using a regular screen top. I would need to see a photo as well to come up with some ideas...