| I think the best silicon to use would probably be "Selleys glass silicone sealant" (clear). It's the only silicon I know of that is made from 100% silicone and can be purchased from the local hardware store. It's water tight, works on glass, is UV resistant and aquarium safe.
Seems like the perfect sealant for any indoor pet enclosures. I think a tube should cost around $8. You'll need a cork gun of course - guess you probably already have one.
Other aquarium safe silicons are available at pet shops but they will probably cost an arm and a leg.
The only other sealant that may be suitable is some sort of "roof and guttering" sealant. I'm sure this type of sealant is UV resistant - don't know about the fumes though. | 
