What I would likely do: Fill with a similar plastic to a size as close as possible. Use a backing if you can, if no backing is possible do not panic. Tape the back-side with painter's tape. Use Testor's cement or, a compatible glue/weld, on the seam first. Let dry two full days. Day three fill seam by pushing-in "Apoxie" brand putty (visit Amazon; I like the gray color Apoxie). This two part epoxy putty is extremely durable. When wet or dry it can be sculpted, drilled, even tapped for screws! Museums use Apoxie for all kinds of restoration work. It is far better than a 3M Bondo Glazing putty (soft) or any modeling putty like Squadron. Apoxie is water based, three hour set, 24 hour full cure. Compression strength is somewhere up in the 2,000-3,000 PSI range, MOE characteristics are also excellent (MOE is modulus-of-elasticity aka the ability to flex just a little without cracking). No fumes. Water clean up. However, wear disposable vinyl gloves and follow instructions on package.