For rigid to work, and I gave it consideration, it would be too bulky and require the tables to be too far offset.
You can see how the velocity is reduced around the perimeter, especially close to the opening. Friction.
Info about funnels is really hard to find. I found that for welding funnels, big units, used for zinc coated metals, pull air in at 2000 ft/m at the opening they don't mention volume. But a dinky little torch is nothing like an arc welder, as far as pollution (for lack of a better word).
For biological hoods, and this, again, is nothing like good 'ole Encephalitis virus, the opening has to be 90-70 ft/m
Neither of those was much help.
In our real world.. we torch with the door closed down to about 9in, I use a block of wood to make sure nothing gets crushed. the side door behind my wife is half-opened.
No smoke can escape from about 2 inches outside the funnel opening and if you stay inside the 8in diameter, between the opening and the torch tip, it pulls the ashes off incense sticks.
I feel safe.
When I get a chance, I'll get the wattage the fan pulls. I have it plugged in to a dedicated 20Amp.
All the lights are 100w equivalent bright white LED
at 18watts ea. I run 6.. really bright.
EDIT:
Here's a link to a supplier of ventilation systems, with specs.
http://www.sentryair.com/index.htm