This is a rubber carpet, grind it become rubber powder and mix it wit cloth, just keep the composition balance, I used 50/50. At bottom you can fill with full of rubber powder to maintain it keep steady.
My bag at home is filled with old clothes. I did not even bother cutting them up, just jammed them in as hard as I could. Weight is not an issue and if you just put in some more clothes every time you feel it's getting too soft the bag wil do very good.
My experience with the sand filling...Had a bag like that in the gym. I boxe a bit, wasn't very hard at the top...tried a hook to where the liver would be and couldn't use my hand for a couple of days. Sand is going to settle and the lower part will be like concrete when you hit it.
If you want to avoit swinging, try to attach some heavy rubber robe (like the ones they use for securing baggage on bikes) to the bottom of the bag and the floor.
I'm upgrading to a 6 foot bag and have been looking for ideas on how to stuff it. I have a standard canvas Everlast bag now with cloth insulation and sandbags. The sandbags drifted to the bottom so I restuffed it a couple years ago. When I did, I used some thin foam laminate underlayment (looks like the roll pic above) to wrap the sandbags into a tube shape. I used duct tape to tape the ends of the underlayment (so the sandbags can't drift around) then packed the foam covered sand tube in the center of the bag with stuffing all around it. It works great and I plan to so something similar when my new Banana bag shows up.
mm-dmoney (first post , long time lurker)