Whats the best way?
 

To rivnut into grp? I have searched for grp rivnuts to no avail! the only thing I found that may work is plastic rivnuts but I can only find them as part of a kit for £60! and I only need about 8.

Is there any other way?

Cheers
Craig

washers on front and back face of grp?

Hmm tried that already and the rivnut was not wanting to grip properly, maybe the washers were too thick I will have another go with thin washers.

John my dash is going to have to go elsewhere as left 8 messages for him now and phoned him over 15 times!!
Think I am going to try autoflek

That’s surprisingly poor! Did you establish which firm Ric H used? I guess the problem with the washers, is that the GRP is already quite thick, so with the washers the expanding section of the rivnut ends up in the middle of the grp. Could you not bond in some flanged nuts?