With how much this is asked I'm surprised I haven't seen this solution provided anywhere. My example here is filling a 250 bag to 150 lb, so its tight without any extra fabric or the need to tie anything down.

Short answer: use two filler materials (like rubber mulch and sand) and keep them separate using a filler bag.

You can also do this the opposite way if your bag is only a little too heavy! Fill with sand and use a filler bag of rubber or foam or towels.