Ok, my vague idea for the tank was 2 4' pieces of 6" pipe, slung under the tractor somehow. Anything that makes the tractor look more like a fighter jet has got to be worth doing. That'd net around 11 gallons, if you could make them fit.
Only problem with the pipe idea, is 6" steel pipe is probably around $20/ft retail, not cheap. Wall thickness is also kinda overkill for the application, but extra dead weight on the tractor not really a bad thing.
Aluminum pipe in that size may be available cheaper, I got some 5" Al pipe on Amazon recently for a good price. It'd offer increased cooling, but if you ever bottomed out on it, high chances of cracking it open.
Failing all the above, welding up a steel, rectangular tank that fit under the machine seems annoying but do-able.