There is a root cause here and I highly doubt that it is because a smaller stud is not tightened enough. First a caution, before just tightening down on that stud, look at all the photographs of broken Tee slots on lathes and mills and other machines. Over torquing is a SURE path to a broken Tee slot. DON"T tighten it to it's yield point: that will break it after two or three such tightenings. NO bolt or stud should ever be tightened to it's yield point unless that is specifically specified by the design engineer and then it is considered a single use bolt and is thrown away if loosened.
Try to find that root cause. I highly suspect that it is a high spot on either the compound's Tee slot area or on the tool post itself. Such a high spot would act as a pivot point and would prevent the outer area of the tool post from contacting the compound and stop the rotation. I would check for that first.
Remove the stud and Tee nut. If the stud will thread all the way into the hand held Tee nut or at least 3 or 4 turns, then it is probably OK and I would not worry about that.
If you must take a make-shift measure, I do like the aluminum gasket idea. I would use aluminum foil as it will not compress much over a period of days or weeks. And I would cut out the center and use only the outer 1/4" or 3/8" periphery for the grip area. This should overcome a moderate sized high spot in the center.
For the long run, I would make sure that the top of the compound is flat and the bottom of the tool post is slightly concave. That will ensure that it rests on it's edges, not on a central high point. This alone should prevent any rotation. You should not need any aluminum gasket or need to over torque the stud.