I don't understand... Whats the top plate?
I have used corroplast to make an extension from the floor to the ceiling for heating. Can you extend your ceiling fan to the floor?
Here in the US we build structures with vertical 2x4 wall studs on 16" centers, they are nailed to a 2x4 bottom and top plate, then a second top plate is added on top (overlapping) to anchor all the adjoining walls together.
The plan is to use the 14-1/2" cavity between the studs as duct work in the wall (behind the drywall), cut a square hole in the bottom of the drywall and add a grill over the hole. Then you need to cut holes in the top plate so a 400 to 500 cfm fan in the attic call pull the air (fumes) from the grill near the floor behind the printer and blow the exhaust up thru a roof vent.
My old ceiling vent was not working because it was not very powerful and it was in the ceiling, the solvent fumes are heavier than air, so it was not doing a good job.
I am a bit of a projectionist, to many years working in the graphics industry doing catalogs and brochures were your artwork would be reproduced 10,000 times, so everything had to be 100% perfect. I don't want to make a coro box hanging down from the ceiling with duct tape, I just rather do it right and have it look clean and professional.
Just like my printer is on the opposite side of the room from the
computer, and instead of running the cable across the floor, I installed wall jacks in the drywall on the wall behind the printer and the opposite wall behind the
computer, then ran 50' of CAT5 cable up the wall cavity, across the attic, down the other wall cavity, all you see is the cable from the
computer plugged into the wall jack behind the
computer and on the other side of the room, the printer cable plugged into another wall jack, clean and professional looking, no cables on the floor or draped on the walls.