It's generally not considered a good idea to put a fuel cell inside a top case. You need a vent/overflow line that would require a hole in the top case. If you're simply wanting one to mount on the rear rack instead of a top case, that's pretty normal.
Boyd Welding has a nice 12x8x8 3 gal tank. You could probably get them to put baffles in one, even though they are not doing one off custom tanks right now.
It might be worth a try to contac
t Rhodes Race Cars and see if they will make you a custom tank. Jazz and RCI 3 gal cells tend to be tall cubes, more suited for a tail dragger application than top case or pillion seat area.
Other potential sources -
RCI Custom form SP Sheet Metal has a build your own tank feature on their website. Handy with different shapes to choose and you input the dimensions and there is a button to calculate gallonage. Interesting for playing with ideas.