In which case the BR-2 is probably your best bet - primarily built around the idea of the secured weight. You could probably do something similar with a soft pack - if you can pack the sand into a fairly rigid container so it doesn't flop, keep the bags upright and fill the surrounding space with padding/filler materials, you'll achieve basically the same effect as the BR-2. If you're still thinking about the RUSH series, the 24 is a little more versatile, depending on how much other stuff you want to haul. The 12 is smaller, and therefore more specific. I can't vouch for how well it will hold up under 35 to 50 pounds of sand though.