Heat and bend. You can connect the edges with a plexiglass glue, or just heat them together with a hot iron, like a soldering iron or plastic welder.
You'll have to ground your parts but it's easy to just jump a wire from the different mounting screws, and then ground it out to the vehicle.
You can see some of the stuff I did with plexiglass in my worklog.
EDIT. I just added some pictures to the grounding job I did on mine.