).BUT, if you want to use the flat lid, you have to also replace the lift arm. Reason is that the little "wheel" that slides into the slot on the lid (it's the little wheel on the end of the lift arm) is on the opposite side of the arm. It's a two screw job to replace the arm, so that's not a big project, but the flat lid won't work with the lift arm for the domed lids. However, it will work on any of the Fantastic Fan bases that use a lifting lid (with the correct arm).
I tried a Flat lid on my original Fantastic vent because I was hoping it would reduce the wind noise caused by it being so close to the front of the overcab. It didn't do much though, and although I liked the sleekness, it wasn't as insulative (I have the "double" domed lid), and it was much floppier in the wind. I would guess it is also floppier than even the single domed lid. Not like it's going to break, but it would "warble" in the wind much sooner.
I also have used both the domed and flat lid on a low profile Fantastic base (which is what I am running now - I like the lower profile on the roof). Same comments though.