Hi Dave
I am no expert, but what I did was used was a piece of formica glued to door skin paneling so it is flexable and take the shape of the trailer when screwed to the inside . bevel the edge of the fibreglass about 3 inches then lay 2 pieces of mat . One 1 inch bigger than the hole ,the next 2inches bigger .when the glass kicks off you can pull the paneling support off the in side and lay a piece of mat about 4 inches bigger than the hole this makes a strong repair
The formica should be smooth , you can also wax it for better release but I found it did not need to
Than fil in screw holes ,body work and
paint. It takes a bit of time
Hope this helps Ken