[brucepick]

I really like the idea for two sliding panels suggested by tasdrouille above. Slide horizontally, that way you're never fighting gravity. Only a couple inches movement will take you from fully blocked to fully open.

However I just recognized this: if using two sliding panels then the holes will be (maximum) one half the available area, assuming the ability to close the slats completely. So maximum opening is 1/2 the original area. If you use three panels then max opening is 2/3 the original full area. However you'd need some clever mechanicals to control three panels.