As D said, Bondo will probably work for you.
It doesn't shrink and is ready to paint fairly quickly.
Try to scrape out as much of the paint in the dent as you can
so Bondo will stick directly to fiberglass or metal. It's a great product
but you only have a short window of time to shape it and smooth it.
You have to wait until it firms up enough so that it holds its shape and
doesn't stick when you touch it. If you let it set too long before rough shaping
you will have a lot of trouble because it gets really hard!
I use a shurform plane to get it down almost to the finish surface. Then
I sand it with a vibrating sander down to flush. Hand sand a little with
fine sand paper then prime and paint.