Looking at the photos Rick, is your rotary table 4" or 6" face? I like that it has a 90 degree mounting flange. Literally doubles the utility.
My work around is a Ebay find many years ago.
A1 5C INDUSTRIAL COLLET INDEXING FIXTURE (kalamazooind.com)
My rotary table has I think a 12" face. It was nothing to muscle around in my 50's. These days? Ha!
In 2004, I wanted a bow casting seat for my Boston Whaler Cohasset. The OEM seat post mount was cast aluminum. Must have been a very limited production item. Hurricane salvage yielded nothing.
To my good fortune Fisher Pierce the makers of Boston Whalers had archived dimensioned drawings on PDF. The original seat post mounts were alum. castings.
Were I smarter than I never was, I would have made a wooden pattern and had castings made. But there is always a balance of Time, Material. and Labor. VS the market demand.
I bought 4"x 6" 6061 enough to make 4. Cut enough material to make two seat post mounts, one for me and one on spec.. Good guess on how many to make. I fitted one to my boat and it took six years to sell the mate.
Sadly all my photos were posted to photobucket, and also reside in hard drives of earlier computers.
Tom