Veiled females get big too, I know people recommend that size sometimes but I wouldn't keep a girl in anything smaller than maybe a 24" x 36" cage, especially since you need room for a laying bin. Female chameleons will lay eggs regularly even without a male, like chickens, just in case you didn't know.
What about building your own cage? If you're on a budget you can get the veiled now for that cage you have now and build yourself a bigger one for a month or two from now. DIY is usually a lot cheaper, and you can build it however you want. Just an idea!