As it happens, I also am in the market for a couple of steel outboard tanks. Gasoline is not to be messed with. A plastic tank is an explosion looking for a place to happen. A steel tank is the optimum solution. Aluminium can be welded up but there is a liquid compound that is swirled around inside before use. Find a supplier for home built airplanes, they will stock it. According to some home built pilots of my acquaintance, it is renewed on a regular basis.
For what it's worth, even a manufactured tank for diesel fuel cannot(?) be rated for gasoline. Some years back, "we" had a large tank built for a bucket truck. An older truck, the mileage wasn't that good. It was cheaper to use a larger tank than turbo-charge the diesel. The fairly small stock tank was to be used for a gasoline "pony" engine to run the boom so the engine and PTO wouldn't run un-necessarily. The gasoline leaked out. It would hold diesel, but wouldn't hold gasoline. Had to use a boat tank.
Modern "gasoline" is not true gasoline, it is called "motor fuel" for a reason. There are so many chemicals in it that it can only be called such. In the deep south, we are forced to use a mixture that is 15%(?) alcohol. It eats fuel hoses and does other undocumented damage to an engines. My personal experience has been very unsatisfactory with jury rigged and plastic tanks. Including Wife's favorite pick-up, which caught fire on the road. It only had 42K miles, but was 10 years old. Ten years of that funky "gasoline" literally dissolved the fuel system and it went up. She knows well how to handle an emergency and got out in time. But the truck was burned to the ground.
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