There's a product called Kitty Hair that works wonders in situations like
this.  Should be available at a boat supply store or some auto places.
-jim tremayne

It's called that because it looks just like a puked up hairball.  It's a mix
of fiberglass resin and shredded fiberglass.  It goes on like bondo but the
fibers give it real strength.  It's also sandable and will give you the best
finish.  Putting bondo on a semi flexible surface and driving through summer
and winter may make the bondo patch suddenly become noticeable.  I'm not
sure if there's a plasticizer (made up word for an agent that adds
flexibility) or not.  I'd go to the local boat store and ask the folks
there.  I have used kitty hair on subwoofer boxes to get that smooth finish
and it's great stuff.  You should be able to find it in a pint or quart can
with an evil looking cat on the front.  I would use long strand and some
fiberglass cloth strips for large gaps and short strand for the little
touchups.
http://www.handsontools.com/store/show_product/?product_id=9625
-jim tremayne