Thoughts on garage venting idea?

MaxQ, generically it is recommended to not have a direct connection to the FEC vent. I would recommend the use of an exhaust fan; these are available in side-vent configurations as you mention. It is best to create a vent hood above the FEC with the FEC vent inside and above the lowest part of the hood to contain the smoke. Then switch the fan on when needed. The exhaust fan will get rid of the smoke but because the fan is not actually vented directly to the FEC will not affect its operation; I would think a fairly small fan would do the trick. You might check with your home insurance holder to see if they have any specific requirements or concerns.

Connecting a vent extention directly might work much of the time; however it would most likely affect the operation of the FEC in windy conditions.

My restaurant FEC sits outside with a metal canopy overhead. I have a 10" long collar on the vent with a 90 degree elbow pointed down, to prevent wind, rain, snow access. It's never created an issue I'm aware of. My primary concern was the 2' lenght of venting + the wind cap. I hear what you're saying though and will explore the fan idea.

go to a fireplace store that sells pellet fired heaters and get a side wall mount high wind vent in 5" size and use that to build the plywood around.

This is what I have on my trailer walls for my dual FEC's
Direct Vent Sidewall

David, is that directly connected to the FEC?

I have a VacuStack cap on mine. The higher the wind, the greater the draw.