the t.bone Ovid System Cello Clip

It works well, and is easy enough to attach.

Note however, that it can have quite an impact on the tone, as it completely mutes the afterlength of the C and A strings, and also changes the resonance frequency of the whole string/tailpiece assembly. Probably even more significant with a metal or carbon fiber tailpiece, which will normally ring quite audibly. That said, for scenarios where any of this really matters, one should probably be using a large diaphragm mic on a low stand anyway.