You can use any kind of ball like this one or a styrofoam ball from a craft store. I just had this one on hand.
I put the walnuts as close together as I could. then added some moss from the dollar store with the glue gun just shoved it right in the cracks with a pen..glue shove glue shove till it is all filled in.
It goes great on my shelf in my bathroom..
It cost me about 1 dollar to make, now that's what I'm talking about!!..