Cervelo p2c saddle cap

Just an annoying problem I havent been able to solve - anyone got a good solution to stop the cap that covers the second saddle hole on the seat post from rattling free. Everytime I tighten it up it just seems to rattle free and eventually fall out!

It’s been happening recently to me too.
Two or three times this year, but not once in all the previous three years.

Don’t have a P2C, but am familiar with their seatposts. Could you wrap the round section of the plug with elec. or teflon tape to very slightly increase the diameter, and therefore tighten the fit?