wing chun iron fist/palm bag

I have recently recieved a 3 sectional iron fist/palm wall bag and I am now looking for ideas as what to fill it with. The site I got it from says plastic bb's or sand to help the life of the bag, I have started to fill it with plastic bb's but they sound and feel odd.

For those who use this bag or have used this bag, what do you suggest to use that is both economical and practical for a beginer?

Also how full should each bag be? Full to the top or should there be some space in there for the media to move a bit?