Obtaining or making a Sandbag
Making a sandbag is easy, or you could buy one online. I made 2 of mine for about $30 a piece.
thick canvas bags at the army surplus store
cotton string laundry bag
strong zip lock freezer 1qt. bags
Fill the zip lock bags about half way, take out all the air out of them, duct-tape the them closed, then double bag them again.
Put them into the laundry bag, then put the laundry bag into the canvas bag. You can also get a nylon laundry bag as well and put the cotton one into the nylon one, then put them all into the canvas bag.
I also made a ton of extra zip lock bags weighing about 2 lbs each. This way I can adjust the weight if needed.