This is what I did to fill mine. I used a 4 inches wide pvc pipe and filled it with sand. I had someone hold it in place right down the middle and filled around it with pieces of cloth. You can buy cups to close the pvc pipe both ways so the sand doesn't go all over the place. They are pretty cheap.
Make sure you stuff the cloth around well. It's a very easy process if you have someone to help you hold the pipe in the middle. The result I got was great. Not too soft on the outside and quite heavy overall.
If you need any help let me know and I'll try to give you more information.