If you can buy a Ronco off the shelf one it will be MUCH cheaper than a custom one however. Smallest is about 6 gals
Here's the 85 page catalog.
Unfortunately you have to go page by page and they are NOT in...
Barr plastics locally. Polyethylene https://www.barrplastics.com/box-shaped-plastic-tanks.html#.YyySJHbMJPY
BUT be sure to use a 2" seacock for the drain. Very important to get it to drain easily (learned this from Beneteau factory). RV dump valves are 3" and they empty very easily.