Just about to fit a rollbar and refit my Avon bodywork. This leaves pondering what to do about the GRP cutouts where the rollbar goes through - fill them in somehow with colour matched GRP so it all blends in, or just leave a gap with some nice trim? I'd like to do the first but not sure how to get a nice result and I worry it would crack as the GRP vibrates.

Anybody have good experience of this? Thanks

I think you're right about trying to fill with GRP...you might try splitting some rubber hose and sliding over the edge of the hole to form a coaming. The right size hose should make a pretty tight fit...

How about using a rubber close coupling washer? Well one for each side.
Look ->
screwfix linky
You can buy them without all the gubbins.
I used them a million years ago on my Robin Hood.
Ian