i got a cheap 5 gallon cart sprayer to hook behind a lawn mower.
i will pull the pump, spray wand off, and a couple fittings and hoses, and put all onto a 55 gallon drum. (check Craigslist) you can normally find the drums from 5 to 15 bucks.
a few fittings and little bit of time. and will be able to have a 55 gallon sprayer. the pump will not be the strongest thing. but good enough. for some spot spraying. and reduce multi trips to refill.
most likely will just put the drum in the front bucket, and put a couple straps on it. and pickup a cheap longer set of jumper cables and splice onto the pump.
will also pick up a hose, fairly cheaply so i have extra length. to reach back to seat. and/or allow me to go a few feet away from tractor if need be.
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i have debated of just buying the "on demand" electrical pump, and everything separately. so i could a little more power. but the 5 gallon cart spray for lawn tractor was extremely cheap. and figure ah why not. for amount i do need for it. it will do the job.
atv sprayers that fit on back of them or into tool cart box in front of them. you can find them fairly cheap as well. you do not have to have them on an ATV.
3pt carry all. you can get fairly cheap if you bid your time and wait for a descent deal to come around as well.
every now and then, i see a bike rake or mini storage rack like what whitnel shows in his picture that you might be able to toss a 55 gallon drum on. or atv sprayer on. and fasten down with a couple straps.
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the problem comes down at times, is some times is cheaper and less time consuming just to buy a small package sprayer or like. and doing some modifications to it. longer hoses, bigger tank, how it attaches to tracktor or vehicle, etc..