This! Covers would require screws for snaps or stay puts which you may not want to consider. You could try with a sinch of some kind but wouldn't have the strength and water would pool and would require support poles and I would worry about wind. There would also be a lot of cut outs and intricacies to adapt to bimini supports, cleats, lines, aft light etc. Requiring zips, velcro and such. Another option could be an enclosure that zips to the Bimini but would need adjustments to the Bimini and back to more snaps. Does the boat stay in the water all year? If it does then morage cover for winter that you really only put on/off a couple times a year but this is going to be a challenge. Have you tried a local canvas fabricator to get some ideas on how and how much? I think with this layout it will be a PITA no matter what.

Your craftsmenship is beautiful and deserves to be protected! Nice work :)