Hose end sprayers are not a good idea for this and are not recommended for Roundup use. The ratios of water to product would not be correct, and they would apply far too much spray too quickly and too imprecisely. There are some small pump sprayers that can be used that might be easier on your back.

jay is sooooo right! This is a perfect example of how to abuse RoundUp, as a matter of fact. The finest of mists should be applied to the foliage of undesired plants, and nary a drop should touch the ground.

The hose end sprayers should be reserved for products where precise measuring doesn't count for much, and where non-target application isn't a problem. Soluble fertilizers, for example.

Sorry, I don't remember where ... but I'm sure I saw an ad for a small (2 gallon or so) sprayer with wheels. If you cannot locate that I'd think some sort of cart could be devised. Large wheels work better over rough ground so luggage carts for airport use wouldn't work well. Hummm, just occurred to me an old golf cart would probably work well and could easily be found at a thrift store or yard sale. The one I'm "picturing" would accomodate a 2 gallon sprayer because the upper bag clamp could be moved lower to allow strapping the upper part of the tank.