What about custom cutting a piece of rubberized mouse pad to lie flat on top of the charging tray? No metal in pad means nothing to activate or get activated with the auto-on charging tray? While laying metal object on top of that may not be a good idea, there is probably a sufficient rubber thickness that would effectively insulate between lightweight loose object(s) on top and the tray underneath, even metal--although that's beyond my non-engineer education. (Or, wood is also a good insulator; more complicated perhaps to cut a piece to fit with a coping saw, band saw, or fine-tooth table saw.) Rubber coating should also hold loose objects from sliding too much. Inexpensive. Easy to remove to clean. Can mix/match with the design and/or colors on the mouse pad to suit your taste.
