This is my Idea for the competition:
Use plates made of spring steel. The plates are attached to the rotating regulator. Plates are formed with hot bending the way that one part is straight, and another has the same radius as the main body wall. Then the plates are attached to regulator. When you turn the regulator the plates are forming the gap, the bigger the gap more air can access.
The first picture shows the plates in the half-turned position.
The structure is very simple and reliable, It can be mounted onto any deck regardless of deck thickness.
Spring plates: spring steel
Regulation handle: Copper, wood
Guide rollers and sliding ring: copper or plastic.
Th rest can be made of brass or stainless steel depending on what is cheaper, or easier for manufacturing.
There are couple of details that need to be discussed or changed depending on manufacturing process and tolerances (for example, spring plates are likely to be covered with something flexible (rubber?) in order to press nicely when fully closed. External air inlets can be increased almost to any size so more air can come in, cut-offs on inner panel are likely to be smaller so nothing can be seen from inside, design of the lid can be adjusted, the handle can be done in any form etc.), but the main thing is the idea. I will try however to make couple of slightly different versions and post them here later as well.