I wish I had access to my 1980s copy of Friedman's book on Royal Navy carriers. He would have the answer. I tried the dubious web, and found a lot of confusion with one informant who declaried that the giant holes in the deck were yawning open at all time, even when airplanes were landing. Somehow, I find it barely credible that the RN would have risked having an airplane stall into the uptake, and cripple the ship instantly. I did some more online digging, and found a WWII photo of the flight deck with a cover of some sort over an exhaust hole. I have no idea if this was canvas, or something study enough to withstand an aircraft impact.

I understand Friedman has a new book incoming on the RN carriers of the two world wars. The answer will be there!