I used to make a craft where you wrapped a BIC ink pen in floral tape and then take the end of the cap plug out and hot glue a fake flower head in it...then you put it in a tiny pot containing pinto beans (kinda looked like pebbles)and I know they held in position....but of course this could not be mailed. Just a thought.
Debbie

If you want something to hold the sticks in place inside the pot that looks like potting soil......why not just bake a chocolate cake in the clay pot....then rough up the top to make it look like loose soil... then stick in the sticks... Lot cheaper than canned foam.... and definately edible.