The biggest weight in a trolley is the pump box. You can get a smaller battery (Or 2,.. its good to have a spare with the smaller ones!) and make your own pump box out of thin plywood or something similar. I made one and put it on an aluminum collapsible sack truck (A draper one, very lightweight). Just hold the water drums on top of the pump box with bungee cords tied to the trolley.
The other option is just a backpack on the trolley,.. maybe an extra water drum too.
Not pretty, but as lightweight as I've managed so far.