See the following web page for a great idea:
 
http://jellostore.archway.com/cgi-bin/cornerstore.storefront/3ff0509306f76630271 7d8f593a2070f/Product/View/93199
 
We have used one of these before and it works great.  Just make the jello a bit thick and it does not leak!
 
If that works, then cement should work even better.  Just spray the inside of the mold with Pam or some such, close it and pour in the cement mix.  At $3.50 plus shipping each, that's way cheaper than any mold I have found on the web and you get six stones at at time.  This is easier than the tennis ball method also
 
Yeah, I know they are egg shaped, but surely that won't matter all that much.  Just always load them the same way in the pouch and you will know how to adjust for thier particular way of flight.
 
This is on my list for warmer weather when the cement will set up right.  
 
Jeff <><

I got x2 of these for $7, and they didnt charge shipping!   Just made a single plaster-of-paris "egg" to test weight, ....which comes in right at 3oz.    I figger to mix in vermiculite or similar to get weight down by 1/2 or 3/4oz   The egg shape is pretty symmetrical end-to-end.   No "fat" end like a real egg.    FYI       mgreenfield

Man this is an awsome idea. I might try it some day when i'm feeling bored. (i.e. in the very distant future