I dont know the mk3, or the differences with a ' grey' import. I initially thought the problem was just the absence of hinge points on the seat back and hooks on the hydraulic struts for the lifting cords. I was going to suggest that simplified hinges and clips could be fabricated that would suffice.
But from the photos it seems the import has significantly different inner boot trim panels lacking side ledges that support the closed shelf. The correct Mk3 parcel shelf would not only lack side support but may also leave a significant open gap each side.
Plan A .obtain inner side panels from a scrapped uk car which would then support a standard mk3 shelf. .But this would be a big job, with no guarantee they would be a straightforward fit ,such as round the boot opening and seat back.
Plan B .Obtain a parcel shelf from a bigger car and cut it to fit the extra width ,with maybe simple side support blocks of high density black foam stuck to the inner panel sides with double sided trim tape.
But extra shelf width might not fit through the hatch opening or roof line as it lifts.
Plan C , Use a standard Jazz parcel shelf and make much more sophisticated side ledges out of high density foam that also fills any gaps.
Plan D, Just an idea, thinking outside the box. May not be practical. Instead of a parcel shelf try to find some sort of roller blind that can be fixed across the top of the seat back and pulled forward over the boot space when required. One with springs that automatically retracts maybe using thin bamboo slats etc covered in discreate fabric rather than fabric alone