on my '86 Tartan 40 a few years ago with new poly tanks. I made template stations out of cardboard every foot or so to get he maximum volume, the shape highly irregular, I made up the entire tank out of cardboard and gave it to the fabricator. He spot welded the tanks and I fitted them to make sure all was well before he welded the seams. I wound up picking up 20% more volume. I put all my fittings on top, vent, fill, sender, no penetration below. I put neoprene under the tanks with some air circulation, and foam them in with spray foam and blue board solid foam, and stringers on top tied to bulkheads to keep them tight, my tanks are under the Settees. I have three baffles in each tank, so four compartments, they are about 6 feet long. I put a 6" Beckson inspection lid in each compartment, I can easily get in to each compartment and scrub all surfaces, which I do once / year. I fastened the lids with butyl tape, that I got from MaineSail on this Board. I am very, very happy with the tanks. Still look like new, no taste, we drink the water, easy to clean & maintain. I use electronic senders from Centroid and a CruzPro to monitor levels.

Let me know if you have more questions, or would like to see photos, cheers, John