Just sharing an idea like I did with switch polishing, limited to tray-mount cases.
With tray-mount, plate sits atop PCB and does not touch the case walls. The idea is to insert rubber wedges between the plate edges and the case walls, fixing the plate in place. I have no idea how a wedged try-mount board might sound but I think it’s worth trying.
As to creating the wedges, my current plan is to cut up a rubber eraser into small triangular pieces with a blade. Scented erasers may also be a simple way to make your keyboard smell, um, fruity.
UPDATE1: Size of the gap may be too narrow on many of the exiting cases. But I don’t see that as a big issue because, if this hack is effective, tray-mount plate and case design can change to support the hack if not ship wedges along with plates. For testing purpose, I’m going to sacrifice a POM plate by grinding some notices on the sides to accommodate wedging.