If you want a custom tank for your boat get ahold of Chuck Harsin of Widow Maker boats. https://www.facebook.com/charsin1

You may want to consider searching the internet for fuel cells. I know Diamondback and Classic airboats sell them. They generally are between 30 and 50 gallons. Prices range between $300 and $500. Then you will have to add shipping on top of that.