I played around for years and finally cut open the port side settee and made a flange and lid that is stronger than new. You can see Mr. Shaw designed nice beds for the stock tanks. Yes under the tanks is reinforced. I had a new poly tank fabricated at great expense and trashed it two years later. It was impossible to get the lid to stay welded since there was no access to weld the inside of the seam. Sadly it was useless.
I installed a big French bladder tank and it works well. The new bladder flops over the smooth ends well and the added weight over each side is insignificant.
The only problem is air building up and preventing water from filling the bladder if the filling supply is too fast. If overfilled it could bust-up the settee so it is a job for the captain only. I've racked my brain to figure an air vent. Chime in if you think you have an answer without adding another fitting.
The new tank is much larger capacity than the old tank.
A thin stainless steel custom tank would be nice but why bother? The bladder uses more of the under settee space and is so cheap it can be replaced instead of cleaned. The French have the plastic figured-out, it is tough and has no taste or smell at all. The tank is two part, water bag and tough outer shell.
The lid is a little over-engineered but stiff as can be and lightweight too. It is supported by thin plywood I beams that harken Rube Goldberg as well.
It was necessary to build that way for a solid tank that fit the original footprint- The i-beam flanges must go in after the rigid tank was laid in- the flanges are two part and nest together after fitting past the access hole. Once assembled they tie together the settee and support the lid, sliding plywood berth and any occupants easily.
Starting with a bladder you could cut a small hole, chop up the old tank from within and stuff in a new bladder in an afternoon... That would be my route now for the Starboard side when that 42 year old tank gives problems...
PS there is a a nice cubby hole storage available in front of stock tank area where a door in base of settee or opening in bulkhead from within hanging locker could provide access without the sizable job of removing the cushions, plywood sliding berth base and new engineered lid.
Additional house or windlass battery?