30AGuy,
My wife complained about the exact samething on our 45 Legend rear berth for a couple of years. We finally came across the solution to this problem at this year's Chicago 'Sailboat Only Boat Show'.
The "Tufted Topper" lady (who advertises in Cruising World Mag) will make you custom fitted sheets for your uniquely shaped boat matresses.
We had purchased a Tufted Topper Matress pad from her the year before and she still had the matress pattern 'on file' to use to make these fitted sheet. All we had to do was send her the sheets we wanted made to fit and the thicness of the mattress and she did the rest. I don't remember the price $$ per se, but it was probably around $30-$50 range. So, make sure that you buy sheets with 'high thread counts' so that they will last a long time.
Regards,
James