Let me preface this by saying I don't have any affiliation with BBQ Dragon and the shelf is the only product I have from them. I've had a few folks ask me if I like the shelf and if it causes a gap with the lid. The answer is Yes - I think it's great and No, the gap is so small it doesn't affect anything.
Here's the shelf from the top. It just clips on and is very sturdy.
Here's the gap - about 2mm. The size of the gap will likely be depend on manufacturing tolerances and the roundness of your kettle. I think the folding supports that contact the lower bowl can be slightly bent, if needed, to make it more flush if you want. I didn't bother to do that on mine as the gap is so small. I suppose if you're super concerned with the gap, you could close the bottom vent a hair to compensate. To me, I just don't think it's necessary. The kettles are not air tight to begin with.
After your cook, you just remove it and store it in the kettle. Easy Peasy. It also has 3 utility hooks to hang tongs, brush, etc.
I like the shelf so much on my old 1991 Red Kettle that I'm thinking about selling my other Weber, which is a Performer Model with a stainless steel table (SSP). I find it's just so much easier to move the Red Kettle around than to wheel around the big Performer.