Indeed a slipper clutch is the usual answer to vibration in aircraft piston engines. It is especially critical in engines with few cylinders but with a reduction gearbox; the Thielert indeed but the Rotax 912 also. Though ISTR only the later 912's have one.
Mine certainly has one, too, and I have had reason to be grateful for that when I milled the prop to pieces yet the engine came to no harm.
Some smaller engines, especially car conversions, use belt drive, which serves as reduction gear and as a damper all in one.
I never heard of using some block of rubber as a vibration damper, curious as to why - it seems a viable idea.