Ultra Fitness Gear Super Tough Fillable Neoprene Pancake Sandbag for Full Body Workouts (Unfilled), 10-50lbs.

I filled it with sand from the hardware store which came through the zipper when I used it. I then dumped it out and put washed gravel in because I figured the dust from dirty gravel would also come through. It appears to be holding now.

Well made, prefer this size and shape as it's easy to handle