Isolating bookshelf speakers without stands?

My smaller listening room is so small that I have no flexibility in where I place my Quad 12L bookshelf speakers. They have to go on the ends of the same desk I sit at while working at the computer. On passages of low-frequency music especially, I can feel vibrations coming from the cabinet through the desk.

All the Quads have, like most bookshelf speakers, is four tiny rubber pads to separate them from the surface they sit on. My question is what reasonably inexpensive product I could use to achieve further isolation: sawed-in-half squash balls or triangular points? Or are they only for components? An isolation platform of some kind?

Any suggestions welcomed with thanks--