Hi Chris glad you found something close to home! I would consider using epoxy for the tank but of course can be done with poly using vinylester as I have heard that vinylester will not react with ethanol just in case they ever add it to diesel. I would form it out of one piece bi-axle cloth if possible though in most cases you will have a seam and that ok but and Use plenty of baffles for strength before adding top. I purchased the kit from AB on my Duffy and added extra baffles then gel coated the inside. I then wiped it all down with acetone till I was sure I remove all the gelcoat wax. On the top I had a ss plate made for the pickup and returns, then glassed in blind nuts to screw to. This gave me a port to clean out if I ever had problems but you could also glass in a thick square pad to drill and tap for your pickups and returns. IMO the next most important thing is how you mount it into the hull depending on shape. On the Duffy kit its square and molded into the shape of the curved side. This allows you to glass it in ridged rather than strap it in so I bedded it on 5200 globs and left no holes to get water in or out as there is only a small gap left around the entire tank making condensation less a concern since its airtight. On a long rectangle tank I would hesitate to glass it in due to flex issues. Though I do not agree with a lot of his stuff, Pasco has a good article on tank mounting. Though it does not apply to your transom mount, if you deck over the tank, again take care of the load put on top of the tank if it’s that close to deck frame to get the most gallons you can. I used a 1/2" gap of 5200 under the deck framing also to absorb some flex and prevent abrasion. Test to 3lbs or fill with water before installing. Having said all this, it took me 2wks to build mine and probably would purchase the complete finished tank from Duffy if I had to do it again and could afford it as it’s a lot of work. I added a center tank a couple yrs ago and had a plastic tank built that leaked in 6m but the FRP tank I built has had no problems. I have put my Duffy through some heavy seas tuna fishing for the last 13yrs and all is well.