I have a 25 Gallon 12V sprayer. It's an oblong poly tank. I set two cinderblocks in the bottom of the bucket and rested the back half on them and the front half on the front lip of the bucket. Pulled the bucket all the way back and then raised it up until it sat level. Took a few bungee cords and strapped it down around the bucket and it's rock solid. As I the the really steep part of the road, I raised the bucket higher to keep the tank level. When it's full, the weight in the bucket is probably close to 300lbs. My rear tires are filled and I kept it in low and 4WD going down the steep parts with no problems.
I hooked the pump up to the tractor battery but had to keep the rpm's up pretty high to run it full power so I just stuck a 12V battery in the bucket and hooked it up to that.
Beats the **** out of having it sit on my tailgate and hanging my arm out the window of the truck while I spray. A lot more comfortable on the tractor and much better ventilation!
Ron