Funnily enough, i started work on filling mine in last night!
Personally, there is NO WAY i wouldve put fibreglass in there. That stuff you could hardly call flexible!
What ive done so far, is first tack in several nuts into the groove both sides, then placing metal strips over them, weld those in, then a light skim of filler to fill the slight gap which is left.
The reason ive used several nuts, is because i originally put the plate straight over where the clips clip into, but the metal was still extremely low, therefore still leaving loads to fill. Didnt want to risk it. My cars not metallic, so i suppose it wouldnt have been to bad to do if it hadve cracked, but id rather not do it in the first place!
So tom, in my opinion, do it properly with metal and dont risk it! If you can weld (which isnt really difficult, just get the right settings, and have a steady hand), then its just time and patience getting the skim of filler right.
Hope this helps,
Al