Well, Twig, as someone already mentioned, you could use a fish bowl. I'm toying with the idea, too, and am wondering if I can use one of my plastic Kritter Keepers, so there'd be no worry about glass in the pond.
One of my bettas just died, so I have a traditional goldfish bowl that is heavy, thick glass... maybe I can rig something up with that.
Cool idea. I had seen them at the pond store, but I'm sure they're expensive. now that I realize you can use other containers, I think I may try to set one up.