I would just clip the sprayer's clips on to the powerpack's battery clamps, and insulate one side securely by wrapping it with a scrap of tarp or something.
How much amperage does the sprayer draw? If it will use up say 1/3 of the battery's AH capacity each session where you use it, then I would add some wires to your tractor's 12v system so it trickle-charges continually while it is in use spraying.
I bought their larger size powerpack that they sold for a while. It works great as portable power for several projects. See winch photo below.
I also bought a small inverter that plugs into its cigarette lighter plug. This can power my DSL modem and wifi hub, and allow recharging my laptop, if the power is down but phone service still works. (Which is what usually happens).