Before setting stones, line the bottom of your lap tray with a piece of leather, mat board, or cloth. This will protect stones from the metal bottom if dropped into the tray --
I also like to cut pieces of corrugated box material to line the
bottom of my polishing machine, to just add a bit of cushion for the
times when your piece gets grabbed and pulled out of your hands by
the buff. I also did this for the buffers at the studio where I
teach, but the next time I went in, they had been removed and thrown
away by somebody. I cannot think of a downside for these cushions,
but obviously someone did.