I put wood pellets in gallon zip locks, then I can adjust the weight easily by adding more bags. I put a pillow in the bottom to lift the weight a bit higher.
A 50# bag of rock salt or driveway salt is about $7, addiional 15-20# bags as desired are about $5...never had the thick plastic bag break. And then, after hunting season, you'll all set for the winter snow
I tried concrete first. But those bags rip and leak and its a mess.
Someone else probably already said this, but sand bags. I'm using a 60 lb bag of course sand intended for ice as that is what I had available. Lowe's sells 40 lb bags for $3.48. I cover it with a trash bag to protect my pack if any of it leaks out and leave it loose so that it will form to the frame and straps.
I use the sandbag as well, got it from ace it’s cement tube sand or something along those lines, bought a roll of
Duck tape along with it and used most of it wrapping it up to prevent leakage. Pretty much perfect size and weight for training.
What I've been using and have been happy with is some cheap dry bags off of Amazon filled with paver sand. The paver sand is wetter/stickier than say play sand so you have less chance of some really fine sized sand making its way into your bag. The big one has 20 lbs. of sand in it, the medium one has 10 lbs. and the small one has 5 lbs. I actually even put 5 lbs. of sand into a gallon freezer ziploc and then put them into the dry bags just as a little more protection.
Not my idea, rather I've been doing the 45-70 program from MTNTOUGH and it is their recommended way of doing it (minus the ziplocs). Allows you to easily get the weight you want in 5 lb. increments and we also use them outside of the pack for things like thrusters, snatch, etc. when doing the workouts. One set of dry bags would let you get 35 lbs. of load and two sets would get you 70 lbs.
This has worked well for me and I like how it keeps you from using hard bulky items that may not sit well in the pack or against your back.