Well it looks like all you need is a vertical moving lever or wheel of some sort to make the vents open and close right? Maybe you can just cut a slit on either side just wide enough for like a little lever that just slides up and down to operated the shut off for each vent, and then just do like you said and rig the vent shutoff behind your existing vents?

 

Yeah this is what I suggested before, but i think its easier said than done...