I've had good results using Plumbers Goop.
http://wiki.answers.com/Q/What_is_plumbers_goop
They also make specialty Goops, for taillight repair and other stuff, but to tell you the truth it is likely the same formula in a different package.
I use it for repairing the bumper end caps on my Jeep and a hundred other plastic repair uses.
You can also use a little fiberglass mat as backing, but in my experience the Goop is strong enough by itself if piled on a little behind the repair where it doesn't show. Just sand a little so it gets a good bond, maybe wipe it down with a good solvent, I use Acetone.
The only real trick to using it is, it takes awhile to set and may tend to flow away from or down the crack or whatever. I use gravity and prop the piece to be repaired up, with the crack or whatever down.
Good stuff, try it you'll like it.
Fiber glass (polyester resin) will also likely get the job done, mixing it is an art. The amount of hardener to be used can vary widely depending on the outside temperature. Too little and it may never set up, too much hardener and it can set up before your finished. If it gets way too much hardener it can get really hot and the epoxy will discolor. It will pretty much destroy any paint it drips on or be next to impossible to remove. I tend to use JB Weld a lot for small repairs, it is easier to use and is not as brittle as straight polyester.
You can also get Polyester body putty with fiber mixed in, which might be ideal for what you are doing. It is fairly thick (does not run much), sets up quick, a little more forgiving in mixing (hardener) and nearly as strong as polyester resin with cloth.
Polyester resin and cloth can be a bit hard to work with, it takes a little practice IMO.