Herman miller armchair shell: Pad building  

rubber feet
I like the idea of having it loose, then it could easily be taken off to be cleaned or wrapped in another fabric 🙂
If you'd like I can send you photos of the several chairs I have with bolt holes. Some just have 4 for the narrow base, and another two I have a few holes towards the seat back for air flow (they were fully upholstered) then you could be sure that the pads would fit any kind of hole configuration
I've done quite a bit of looking (all my chairs are pretty junky and need some sprucing up!) and as far as I can tell, no one is making anything like this!