Plexi glue.... it's in a squeeze bottle with a syringe... you put the pieces together hold them then squirt a tiny bit between the panels and it'll run the whole length! We used it at AI building speaker boxes... works well!!
I'd check the yellow pages for an industrial plastic supply house in your area or on the web. If they sell the plastic they will have the proper glue.
I'm sure where ever you find the plexi they'll have it too. Check a boat place, they sell plexi for the windshields on bass boats!
Google IPS Weld-On Industrial Plastic Ashesives. Most Plexiglass supply stores carry it. If the joint is tight, use IPS #4. It flows like water and sucks itself into the joint. If the fit is a little rough, use IPS #16. It bonds well, and is thick enough to fill in small gaps. We used to build saltwater fish tanks using this stuff and it works extremely well. You'll break the plastic before the weld.
I attached the electrical control center [fuse box, relays and flasher] to a clear piece of plexi attached to the right inner firewall...spaced it about an inch from the steel firewall....think I'll do it again with my 34 coupe.