I discovered this by accident. I bought a dozen of the cheap hollow white plastic practice balls at a sports store and found they were useless. They quickly became unusable, either dented and/or split along the seam. I grabbed one that had almost completely split apart, cut the remaining tab holding the halves together and tried hitting one of the halves. It worked brilliantly! While a whole ball would quickly deform/, the half ball must be springier because it mostly retains its roundness and if any dent develops, you just use a thumb to push it out. To use, you position the half shell on the sensor so it looks like a full ball (i.e. round side down, hollow side up or slightly toward target). It produces a far more satisfying swing than the 'whiff' type effect you get when using the "No Ball" option and as far as I can tell the Optishot readings are the same as I got before using the foam balls .
An unexpected bonus is you no longer need to hit into a net. I hit into a light sheet just to save me walking a bit but the ball halves are so light and non-aerodynamic they don't really go any distance and would be unlikely to damage anything they did hit. The second bonus is that it really helped me stop topping the ball (of course!!) which may have been what was causing me to wreck the balls in the first place. And the half shell looks like it is going to last forever - I'm still using the first one.
Note - because of space restrictions in my practice room I only use irons so I've not tested this with woods.