YSs, Have you checked with
www.boatseats.com
I think they sell different length of seat posts.
Either way if I can manage to do this bolsters custom I will keep all my drawings and pictures and costs. Then you can build some if you like. I know it can be done because my friend did it and they work well because he has them in a 1979 30 foot scarab that he totally gutted and did a custom newer style interior.