Drone -- When I first started driving my hack I used a 70# tube of sand because the woven fabric was a lot more durable than plastic bags, and it was easier to load/unload the hack. With Barley riding with me so often I just got rid of the sand entirely. I do, however, have a couple of Rotopax 1 gallon water containers mounted on the back bulkhead, right behind where the seat used to be. Not much weight in two gallons of water, but there you are.
FYI I try very hard not to fly the hack. Barley is normally a very good monkey, but should a rabbit or squirrel cross the road in front of us he will lunge, and when that happens while flying, well, let's just say I had to change my LD Comfort shorts a few times.
Another option if you have an old unused fuel can is to fill it with sand; that would certainly make the sand ballast easier to handle.
I got a sidecar to travel with my dog. He never complains, is delighted to be with me, approves of my dietary choices, is a social butterfly who helps me meet folks, appreciates a good beer, snuggles better than my wife, and hangs on my every word as if it's the most profound thing he's ever heard. TravelsWithBarley.com