This is how I did my vents for make up air.
I have 3 coming in, all smooth wall ducting.
You don't want to use corrugated as it slows down the air flow.
These come out on the sides of the building and have screens
for bugs and little doors that I can shut when I'm not using the studio.
The vents are at the back, away from the torch flame. I don't
want the air to pummel my flame, this works perfectly. I keep my
air conditioned air and only the fume gets sucked out.
Old picture and old torch, I'm running a scorpion these days,
& my creation station is much more burned up these days, lol.
A big thanks to Dale who helped me design this set up. It works great.