: The 15 lbs that i tested with did so add stability as you and Rob pointed out
: so i'm thinking about the sand bag game.

Sand bags work great if you are putting in from beaches which are primarily sand or small gravel. I have a preference for using 2 liter bottles. Filled, each one is 2 kilos, or a bit over 4 pounds. You can bring along one or two with fresh water fr drinking to keep you hydrated, or bring empties and fill them with ballast water from the put-in site.

Fill the bottles after you take the boat off the car rack, and empty them before you take the boat out of the water.

Stuff the bottles in a mesh bag and you'll be able to hold them in place with some string.
Hope this helps.

PGJ