In the past, I've had a makeshift answer to something just like your problem. I took .22 caliber cleaning patches and wrapped the undersize roundball in it, much like a smokepole rifle shooter would do normally. This kind of works. I do say kind of, because inevitably cloth threads from the patch end up in the darnest places.
One best not be an any kind of hurry using this method as you are constantly removing patch material from the cylinder gap, forcing cone etc.
Edit: Oops! Forgot to mention something important. You'll also need a ball puller for this method just in case!