I decided it was time to make a custom fit sheet!
With the sheet on the bed, I just went around the perimeter and used straight pins to mark the pattern.
I cut the excess material away - 8" from the edge.
Then I made a large dart at each of the 4 corners.
Then I just hemmed the entire edge....and sewed about 2' of elastic (stretched) to each of the corners.
And that's about it! A fitted sheet for a odd shaped boat mattress!
And here it is in place. I'm pretty pleased with the results!