Don't show this to KC Vale, you may crush his love for the peanut tank........if that's even possible.
Pfffttt, the 2.5 L 2/3 gallon tanks with the kits I get work fine when total cost is an issue but I LOVE custom tanks, had a couple 'keg beer' behind the seat tanks made for special builds at about $120 delivered a pop.
I have mentioned before I think GhettoBikes makes great tanks and I love the look.
You guys have the knowledge and skills to design a 'plug in' tank, what I would REALLY LOVE to see is you try working with plastics.
A black plastic tank that would snap into the frame of the common beach cruisers like the Macargi Touch, Pantera, and Fatal Love bikes for under $75 delivered would be great.
As an under $100 option on a new build I think it would be popular.