There aren't many decent resources for this common issue, so here's one.
Tools/Materials: Small cutters, metal plating, epoxy.
Procedure: Measure out the metal plating you need to cut to get good coverage on the part. Cut this out twice. Use the two pieces, one on each side. Do not use only a single strip on the bottom of the trigger, this will not hold. Remember, more surface area is better.
Use epoxy to connect the metal strips to the trigger pieces. Devcon Plastic Welder is good.
You may need to sand/dremel/file down portions of the trigger and/or shell in order for this repair to fit. Test frequently by assembling the shell and trigger.