I have had two custom boat covers made from the sunbrella material. The cover maker uses the boat to sew and custom fit the cover.
A custom cover maker should be able to make a cover with both round grommets, draw string and snap-in grommets.
He places the reinforcement material EVERY PLACE that there will be "wear points". My covers have grommets and a "draw string" around the edge of the cover.
Cords provided to tie from grommets to boat trailer.
If you go the route of having "snaps" consider where they (the snaps) will be placed and the "problem" if the cover should shrink or stretch, how do you compensate for the difference in fit?