You'll probably have to experiment. Vibration isolation works least well when the material is in compression, which you have to do for speaker mounting. Finding the right balance of gasket material stiffness, strong enough mounting screw pressure to hold the driver in place, and loose enough to provide isolation, is not going to be obvious.
Be aware that it is very easy to short-circuit the isolation. If the mounting screw head touches the speaker frame, then it conducts all the vibration force into the cabinet and the gasket does nothing. If you put a rubber washer or gasket between the screw head and the driver, notice that it should be stiffer than the gasket material, in proportion to the ratio of areas bearing load - again, difficult to assess. Next, you can worry about the screw shaft touching the driver frame - wrap that screw, or slide it into a rubber hose of appropriate size and stiffness.
Back when I was playing with speakers, it was not unusual to glue the speaker to the box with silicone RTV caulking. You can, with moderate difficulty at least in theory, pry the stuff off if you need to get back into the box. In practice that's a fairly frustrating task, so try to get it right the first time if you go this way!
