Cage Topper for a 15gal aquarium?

I have a hamster, and lives in a 15gal aquarium. He's doing well, but I noticed some of the cage first a lot of websites. Unfortunately, only seem to be made for 10gal and 20gal aquariums. Do you know where I can get one for a 15gal? I've seen one Ebay, but it was the biggest thing to look .. Thanks for your time! I should mention by Topper, I am not referring to the top of an aquarium. I HAVE to. I mean, like a cage extension laying on the tank top to give extra levels.

Well I had a girl 15 and I had a high for that. It was the black wire with a small door in it. I'm pretty sure I have it at PetSmart. Call your local PetStore around and ask if they have in stock.

Two Russian Winter White Dwarf Hamsters