From Stephen Mayotte, smayotte@mmsl.com:

I built a gadget that does this last year. It's called a "puff box".

There's really nothing to it. Start with a good sized cardboard box--
say 2' by 2' by 2' for example.

Remove 1 side of the box and replace it with some sort of flexible
membrane. I used 4 mil poly sheeting. Some sort of rubber would be much
better.

On the side opposite the membrane, cut a round hole in the middle.
Experiment with the size. I made mine 4" because I needed to charge the
box with a F-100 smoke machine. The hole for a Rosco fogger would be
smaller.

Anyway, fill the box with smoke. Whack the membrane and ta-dah! It's
really easy. Everyone who sees it figures you must be a genius.

You can make a small flap to cover the hole. That way, you can fill the
box up and not lose your smoke right away. You should be able to get at
least 1/2 dozen good rings before needing to refill.