I agree with Alice. If you are going to be using your own rock then put it on the cheapest stuff you can find. When you run out of small rocks to attach it to go to the LFS and buy a couple of pieces of
dry base rock. They will probably cost you a couple of dollars each. Have at it with a hammer and you will have lots and lots of small rocks to attach frags to. You should be able to recover the cost of the
base rock with just one or two frags and you will end up with enough rocks for many more frags.
Andrew