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.
Jim at Jimmy Tarps made me a nice bag for sand bags or meat out of 500d Cordura. It hangs from the top of the pack (Kifaru) by 3bar sliders and is narrower at the bottom than at the top so sand cannot settle to the bottom of the pack. Mine holds about 40 lbs but he'll make it to your specs.
Let a bag of Sakrete set in a really humid area for a couple of weeks. It set up "hard as a rock". Won't shift or flow as long as you tie it down tight in your pack. I also recommend putting it inside a kitchen size trash bag. I used duct tape to seal it up and also tape down excess plastic. All this just in case there is residual powder loose.
Just remember that a bag of sand or water doesn't accurately account for the volume your gear takes up in your pack. I've never understood why you'd train with anything other than what you planned to carry on a hunt, to each their own.
I used Dumbbells. Easy to add up what kind of weight you have in the pack. Easy to add weight as your training increases. I put an old sweatshirt in the bottom of the pack to fill up the bottom of the pack a little . Then roll the dumbbells in a T-shirt. Then adjust the tension straps to hold everything in tight. Worked great.