I have a trolley and am thinking of getting a backpack for awkward one here one there jobs but can't think how I can possibly fill it without taking the trolley out of the car and filling it from that which would make it a waste of time
Fill it before you go to work, if it's for occasional use. One fill might last you a day.
If you're struggling for space in the car, save your money, take another barrel of water and just get the trolley out.