Sorry, that was unfair, I should have provided a solution. Find a local boat repair shop that has the inside track with Moeller. They make ton's more tanks than they show on their website. I'm sure you can find one that will fit your needs and be safe.
flat shallow "belly tanks" get funky faster, too, due to the surface area. I know you are looking for customizing, but 2 6 gallon tanks makes more sense to me in a jon, unless you have a center console to put a 12 behind. Aren't you going to have to raise the floor up so high as to compromise your already low freeboard?
Boyds welding in Florida...they fab stuff out of aluminum all day all week all year...they have done some work for me and it was all custom to my spec...they got it right...price seemed good....they make gas tanks as well...so they are probably certified and will warranty their work
good luck
I personally have 2 ,9 gallon plastic portables. But i store them in a hatched compartment..they can be pulled out and cleaned or thrown away easily. if they ever stop working or get debris in them
bob