i would have thought the effort involved wouldnt justify the ends opening them out for 8" s vs putting some 6" mid dirvers in there.
eitherway way you defiantly want to make some sort of backing to seal them up or theyl ratle and flap about in the open metal space.
working with fiberglass isnt that hard but its really dependant on what you moulding into , boot builds are easy since thers a nice open space and not much going upsidedown , those holes on the other hand wold be a PITA !
if the dirvers where slightly smaller however you could mould the cups outside the car and seal to the mounting rings so it was justa drop in , that would be easy to mould.