Hi,
I can tell you what to Not put in it : sand. It thought that was what the "standard" but it's not. It really hurt when you punch it
Then I emptied it, and I restarted. I put some very old clothes, and some very cheap carpets, and put some sand in top of this, and then some carpets again and finally some sand again. And now it's cool. I think its height is 50kg, and maybe 40kg are from the sand.
I don't with what they stuf the ready-to-use bags but I found it too light.