Mr. J. Hoffman,
Your idea is great, it gets the job done, and done well!
Is there a way to use the gravitational pull/fall of the spark debris within a non-heat-conducting pipe material, in addition to a copper tube heat sink positioned in the center of the downdraft pipe that pulls the heated air up and out of the bucket? The more you create sparks, the hotter the copper tubing becomes, and the more aggressive exhaust of heated rising air will create a negative pressure in the bucket which will suck-in the cloud of sparks. The size and configuration of the copper heat sink will determine the success of this design. A number of medium-sized copper tubes or a coil of copper tubing feeding into one small vertical tube will increase the velocity of air being exhausted. The catch pipe must not absorb heat, but must direct the sparks and heat to the heat sink tubing.
If this works, there may be no need for a noisy shop vacuum.
Thanks for your ear,