Requesting Advice: Setting up first forge flue | For Sale | Forum

Hello. I'm sure this question has been asked a lot and, through some basic searching, I didn't quite fine the answer(s) to my question(s). Please consider providing feedback on the following idea:

The forge I'm setting up (is on a stand and) has air flow coming from underneath and so I'd like to setup a top-draft flue system (as opposed to a side draft one). I don't have a "traditional-looking" forge hood, but do have the ability to cut a closed 55 gal. metal drum in half lengthwise and use one of the halves. From there, I would suspend that from the roof ties. In the top-centre of the barrel's arch, I'd cut a 12" hole and attach D=12" round ducting straight up through the roof of the shop. From one site I read through, it should go 4' above the roof line, ending in a (12") roof cap. 

Questions: Does this sound reasonable? What gauge of ducting is fine (26/28/30)? Do I need the whole thing to be double-walled, or is just the part that goes through the roof enough? (The roof is corrugated metal, btw). Is it best to strap down the nine-ish feet that would stick out from the sloped-roof? What other considerations am I missing?

Lastly, I am attempting to keep costs down, while being as safe as possible as I would very much like for the shop-to-be not to burn down.

Thanks in advance,

~Murray