I have made 2 rolls of my own, but whilst they work well they end up pushing on the brake / gear cables. Given they just hang off the bars there is no getting away from the fact that it will bounce around when travelling over very rough ground. I was thinking of an after market bracket that fixes to the bottom of the fork steerer or to the suspension crown that enable you to also fix the roll harness from the bottom of the harness. This will stop the harness from bouncing around.
I took two old hand grips and cut them into about 2-inch sections. I run straps around the bar, both ends into the grip sections, through a hand made harness, then around the dry bag, just like a typical harness would. And then I take the extra strap and pull it up and back towards the bars, and tie them off to short aero bars. The bag is off the brake cables and does not move at all, even over the roughest surface. I'll try to get a picture the next time I have it loaded. But it's super easy and cheap as dirt.
There's also a harness template somewhere in the DIY forum that looks like it works really well.