I thought of something else ,if you wrapped the carpet around an old 4" plastic pipe and sealed the bottom of the pipe, it would make it easier to roll the carpet around it adding the ropes would be easier too as it rolled you just lay them in and keep going.
With this you could add a small freezer bag of sand occasionally as you progress into the tube, so eventually you have a full tube of sand down the middle and a very heavy bag.
Great for the progression.