Just put some thick tape like elastoplast or heavy duct-tape on the other two feet. It gives the feet more grip than just the hard plastic. Elastoplast (medical tape) is good because it has a rough surface like canvas.
My shifter board is about 2 mm too thin so the third screw doesn't even meet the surface. Some tape on the other two feet and the shifter doesn't slip about at all anymore.
If this doesn't work, you might be able to find a large steel bolt at the hardware store. I don't know the thread size though sorry.