Hey Josh, I was in the commercial sign business for years and manufactured large plastic faces, so I can help you out. The sign is probably made from acrylic, and can be repaired with Methylene chloride, also called Dichloromethane, which is the adhesive manufacturers use to glue panels, hanger bars etc on the large commercial signs you see everywhere. I would suggest going to youtube and watching an instructional video on using it, there is no forgiveness, and it will strip paint off the inside of the sign face faster than you can sneeze, but it's the only viable option. Best of luck, just be diligent, it will be worth it!
