What I have found to work really well for sand is to take the 8" rolls from a vaccum sealer and make a long tube, fill it with whatever weight worth of sand and then seal the end. After this it try to double bag it in another 8" tube then actually vaccum seal it until is almost complete and then manually seal it. Keeps the sand where it's supposed to be and also holds shape really well. You can then make multiple sizes to fit whatever training intervals you choose. The secondary bag is not very fun to do, but once you get it done, they are very durable and I have never had to do it again. I guess yo could use the 11" roll for the secondary bag, wish I would have done that. I do run the tubes in a dry sack just to keep from any friction wear on the pack itself....I would rather leave that type of wear and tear to hauling the good stuff.