You often find in breakers the odd poverty spec mk3 with good arches. Cutting out the outer and a section of the inner arches and fabricate into yours. If done right it could remain invisible if thats what you want. I've done all this work to my last mk3 but used aftermarket repair panels and it was a far job to get right, albeit it was a more door. I jus scrapped a 92 vr6 with solid rears so they are out there. The early mk3's have slightly thicker steel at the arches.