Ok, so I went with the plywood box lined with FRP paneling route. It doesn't look like much, but I'm a brewer, not a carpenter
I believe it will work just fine. Anyway, my next problem to solve is how to route the duct and where to install the fan.
My plan is: on brew day, I will open the window and place a wood board that is cut to fit the entire window opening exactly. This board will have a piece of exhaust duct placed through it, and then it will "periscope" up out of the well. I will then have the make-up air inlet hole below the exhaust duct, and I figured I'd have to attach some kind of (probably flexible) duct to that that will lead inside, and probably on the other side of the room so the exhaust hood isn't just sucking up and spitting out the return air. My question is, how should I position the fan and exhaust duct? Should I mount the fan right in the wood panel that will fit in the window opening, since there won't be much room between the hood and the window? I was also unsure about what fan to go with, and how much CFM. I'm loosely following Kal's (theelectricbrewery.com) design, and he suggests a 450cfm 6'' diameter fan. I don't know if that will be overkill for my system.