For Sig's plastic parts i have always used modeling plastic cement. This will hold pretty well and then i will reinforce the joint on the inside with some CA. I have used the plastic cement as filler before, but i discovered that epoxy with a bunch of micro balloons (or baby powder in my case) does a dang good job.

Then if you do some good prep work before painting, you will never see the joint and it will look as good as any fiberglass cowl.