Gillie - interesting to see seating designs evolve to one similar to what Moesly used on the SeaCraft 21 Overnighter in 1961!
Check out these seats:
SeaCraft 21 I've ridden on the 21' Unohu and those were very comfortable seats that had a ton of storage inside them. The big 3" thick seat cushions were designed to also serve as bunk cushions! The 21' that was for sale up in St. Augustine even had a fold-out 2 burner stove built into the backrest of the rear facing seat!
Fred - Another option, at least for the port side, would be to just try to find a Seafari owner with a galley seat they didn't want. I believe they were standard on the early I/O models. It has room in it for at least one and maybe two 5 gal jugs if you don't use a very big water jug. There's room in the top of it where you could add some tackle drawers on the side aft of the sink section.
The original galley seat did not have an opening in aft side; access is via a large opening in the front. Mine already had an opening rather crudely cut into aft side, so after reinforcing the edges of the hole with a couple of layers of 1708, I covered it with 1/2" PVC foam board and added the square hatch. Denny