There could possibly be other causes, if you are regularly experiencing slippage.
Going by memory, I'm thinking that the Spindle is only threaded so far on the shaft, One could possibly shim the Platter on the Platter-Motor Arbor, thus in effect raising the Platter on the Spindle.
Or one could swap the stock Cork Mat, and replace with a thicker Neoprene Mat. Neoprene sheet They can be gotten in 12x12 size from places like McMaster-Carr. Suitable Shore A Hardness, would be between 40-60.
With the stock Cork Mat, I on occasion would get slippage, if I didn't firmly crank down the knob tightly, but since changing to the Neoprene Mat, I never have that problem anymore.
The big metal clamps might look cool, but there's always the potential of accidently dropping it upon the record. There's less likely damage if this occurs with the little Nylon Nut.
The Rubber Mat, IMO killed two birds with one stone, less liklihood of slippage, and a Mat that is much easier to keep clean, and is non-porous. Mark