strapped down water bags make the most sense to me:

its useful if you get thirsty, or trapped somewhere.

when you camp, you'd put them there anyway, so its typical. (like keep the wt close to the cockpit as possible like you would in packing.)

its easily variable, especially at takeout, you empty and have the least wt to carry. At the extreme you could fill up on putin w/ water also to save carrying.

least likely to damage/stain the yak and clothing.

if shifting damage does occur, it would be way less as the impact is spread out.