The KAPE Seat is an option for sailors who require more trunk support.  The side supports are adjustable in height and width.

The price is $750 for the complete seat  with headrest- $500 if you would like to scrap an existing seat and just mount the new back on it. However it is better to go with a complete new seat as the old seat hinges are fragile and if you need to replace that anyways you're better off with a full replacement.