Christian, it would seem like fitting boat upholstery cushions would be much easier than motorcycle seats. Boat cushions tend to be simply slabs, flat top and bottom, even thickness, and the only question is what the four(usually) long sides will be.
In the garment industry in the US that problem was solved a long time ago, you make a brown kraft paper outline of the existing or desired cushion, called a "maker", and the fabric is then cut to match the maker, plus whatever selvedge is necessary for the seams. Ideally the maker is rolled, not folded (folding stretches paper) but that's simple, cheap, and light to mail overseas.
This is also how garment makers cheat. They crumple up the maker and then smooth it out again, which serves to uniformly shrink the paper, so they need slightly less fabric yardage, and over a thousand pieces, that adds up. (G) Called "shrinking the maker" and the reason why outsourced garments often fit too tightly.
With one set of boat cushions...I don't think a concern.
In the US most folks would think "Kraft" paper has something to do with the cheese company. It is just German for "strong", used in many industrial uses.