I don't know about the 120, but I wouldn't vent a 100 that way. I would want the vent to be higher than the exhaust on the cooker. The Cookshack manual recommends a 4 foot vertical draft for an FEC100. I don't know if you really need that (which would imply a roof vent) but I would think it should go UP and not the U shaped turn you have there.
Got your PM but I'm full time on the road. My worry is that the FE can't push the air out the vent. You might need to measure the CFM and likely add an inline vent extraction fan. Give CS a call, they'll help you out.
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