Our first inboard had a custom stainless tank that replaced the rear seat.... we did put the seat on top of this tank then so we didn't lose seating.
It was basically just a big rectangle that fit under the gunnels. One large pump filled it from the bottom and there was a vent on the top. 1 main baffle ran down the middle to keep things from sloshing too hard. It held ~1000 lbs.... WAY better than trying to deal with bags in a direct drive. Welding in hard tanks to a V-drive seems foolish though since that area is out of the way.
Unfortunately this is the only photo I have that kinda shows it (the metal under then seat) -