You're asking the wrong person
I consider fiberglass and/or resin coated plywood tanks to be unnecessarily heavy, and if you count what your time is worth, to cost at least 5x what you can buy a top quality polyethylene tank for. And unless exceptionally well sealed, fiberglass tanks can absorb liquid. Before you start this project, check out Ronco plastics. They offer more than 400 different shapes and sizes, their tanks are 50% thicker than any on the market except SeaLand and their prices are extremely reasonable. Plus, you can special order any of their tanks with the fittings installed to your specs. Ronco's website is athttp://www.ronco-plastics.com