I load my larger drybags very full, make a loose roll of the excess, and then plonk my lower torso onto the bag to compress excess air out the loose, open opening. From that position,
while laying on the bag, I tightly roll up the excess and seal it with the ladderlock fasteners. This forms the dry bag into a roughly elliptical cross section, and there is very little air left in it. These go into a compartment after smaller bags packed and rolled with less vigor crammed into the end of the boat, leaving some channels for longer loose stuff like tent poles, Sven saw, hatchet, rainfly in a nylon stuff sack (not a dry bag), sometimes a rolled sleeping pad, etc. The result is full use of the space in the compartment, with few voids. More bulky stuff like a cook kit, food bags, camp boots, maybe an MSR Dromedary bladder, gets packed last, with odd voids filled with fuel bottles, water bottles, , etc.
This is slower than a mass thrash with all your dry bags, and some sand or mud gets in there, but it makes better use of the space in the compartment. Unfortunately, smaller circular hatch openings do not accommodate my methods.
PS, others will flock around during the drybag compression phase, making rude remarks, but you have to be thick skinned!
