Well, you either narrow a stock one, or call one of the EFI tank guys and tell 'em what you want.
Or a fuel cell, either in or under the trunk - see what Speedway might have.

When we added wheelie bars to my Duster - after a 300-foot ride on its rear bumper - the right side of the tank interfered.
Cut that side off (2-3 inches, IIRC) & welded flat sheet to the tank. It worked fine, never leaked a drop.
If you're doing both sides, do 1 at a time.