My 2 bits are to find a local marine canvas maker and have them come out and measure up exactly it is you want. “Sunbrella” seems to be the leading canvas brand, but I’m sure there are others just as worthy. The boat may already have snaps or lift –the-dot fasteners already in place for a cover that you could utilize. The fit and durability of a custom cover will most likely be worth it in the long-run.