You'll want a chest strap with a quick release buckle, to hold the main shoulder straps together in the front of your chest and off your arms while you pole. Something simple like
this might work - make sure it is quick release in case you fall through.
Another option is to go to fleet farm, buy a cheap plastic toboggan. Then some rope, a carbineer or buckle, and a fat army surplus belt - make a simple sled harness. Or; use one of your backpacks with a belt to tie the sled harness to - the shoulder straps on your backpack will help prevent the belt from sliding down while you pull the sled. Not fancy, but maybe OK for a short trip.