I wonder if some candle wax might help. Use the wax when mushy, let it set inside stem (or use it when mushy for a stronger fix - removal using a lighter/ focussed heat source on metal to gently heat wax) , and then gently mount keycap onto switch.
Don't know if this will work for these keys as I don't have one but for me it creates a snug fit on other metal keys I have. At worst I use a tooth pick to remove the wax.