I love the idea
I have thought about this myself in the past but never got the project started. I agree with the above about keeping the lid closed but this isn't always possible. For instance, at times I may be cooking different types/cuts of meat that need tending at different times.
My main question and the reason I never started this project is how will you reach the eye bolt if you have a loaded rack above it? At least on my drum there is very little room around the edges. Perhaps a long, narrow tool?
Second, on a smaller level, as drippings fall and dry/cook on the metal, how can you keep it clean enough to close? For instance, grease starts dripping and collects towards the middle. As it cooks off and becomes hard, you will have obstructions to the vent closing I would think.
Still, as gtr said, mad props on your effort and definitely try it out.