Ed,
I run factory ballast... so no advice in that department; however, since every boat is built different and every person has a different tolerance for risk... I would run my own little experiment in the garage.
Fill the rectangular sack, outside the boat, and see how much it deforms when unconstrained. Then put it up against a wall and with a piece of plywood... make it rectangular again. That will give you a feel for the kind of force the side walls will have to endure at least under static conditions.
From there... a rigid container (no mods to boat) or even double up and anchor the side walls (mods to boat) should suffice.
I would imagine that the sack will deform more or less and require more or less force to constrain depending on how much you fill it... not sure if you will always fill sack to the max or not.
I will also look back and get a name of a guy with a 22SSV that added a bunch of custom rectangular sacks and got up to +/-2800lbs... he should know about stress on side walls