Wow, thank you guys all so much for your ideas!!
I've been scrounging eBay (and the rest of the Internet) for a top that would fit, but I think I'm going to end up with something homemade. So far though, I've only seen one light that's small enough, a little 12-incher.
Also, has anyone got experience with filters like these? (Bananas included for scale
) I've got a normal big 20 gallon at my parents and I've used the standard modern filters my whole life, but I really want to go with this for vintage's sake. I was thinking of even tying moss to the front to hide it and to bring more green to the tank.