Before I go any further at all, I should mention that just like the Gougeon Brothers, I am in no way all at endorsing that anyone ever should build their own gasoline tanks, ever! The risk of making a mistake is dire indeed, and what's more, the new alcohol additives in gas these days will actively degrade and weaken many resins, including epoxies and non-specialized polyesters. DON'T TRY TO MAKE YOUR OWN GASOLINE TANKS! EXPLOSIONS ARE BAD!
But what I needed for Phoebe was a small, 4 gallon diesel fuel tank to fit in an abovedecks cockpit locker, and after reading and researching the issue, I decided that a stitch and glue ply/epoxy composite tank had many virtues and not too many downsides for my particular application. The Brothers Gougeon are a bit cagey on the issue of building fuel tanks. It's clear they are being very careful to limit their liability. They offer instructions that start like, "we don't necessarily recommend this, but if you were to do do this, you should do it like this." I agree with them. I'm not recommending or endorsing that anyone else be so foolish as to build their own diesel fuel tank, but I guess it might be interesting for you all to see what this fool did.
These are some of the considerations I took into account before starting out:
*I could relatively easily make a complicated shape to maximise my utilization of the space
*the epoxy/ply walls of the tank are pretty well insulated which reduces condensation problems
*it's a simple matter to make very smooth, well radiused inside corners and joints to eliminate crevices and sludge pockets
*aluminum and steel both have their various issues with corrosion and electrolysis
*I can't weld sheet metal worth a damn, but I have built a bazillion S&G structures
This technique should work just great for anyone wanting to build a fitted water tank, holding tank, buoyancy chamber or whatnot too.
To make a pattern for this three-dimensional shape, I started with a cardboard box and stuck little strips of cheap doorskin plywood to it with hot-melt glue to define the edges and boundaries of the cockpit locker. It doesn't have to be pretty, but you should pick up all of the limiting dimensions with your strips.
I then cut out plywood panels to match the facets of the shape and started tacking them together. For this project I used some scraps of a multi-laminate aircraft birch plywood, which as it is somewhat less rot-resistant is something I wouldn't generally use in boat construction, but for this project which is utterly and thoroughly encapsulated with epoxy and fiberglass inside and out it can't possibly matter. What does matter is no voids! Don't use CDX or any other crappy plywood.
The panels here are tacked together with little stainless brads, but it doesn't really matter all that much what you use. It could be wire stitches or zip-ties or duct-tape as long as you can hold the shape together sturdy enough until the epoxy fillets can cure. The routed slot is for a visual sight gauge of the fuel level--I stuck a little piece of clear lexan in it before glassing over it inside and out. I used quite a few layers of cloth both inside and out and then rolled on coat after coat of resin on the inside as per the Gougeon's not-instructions instructions. I took pains to get the inside pretty smooth.