Since you mentioned the Schleicher TN and the Schleicher TN's list the RAMI AV-22 as a suitable antenna I assume you're installing the transponder in a Schleicher. RAVI's installation instructions on their website give no guidance as to sizing the ground plane as they only refer to externally mounting the antenna on a metal airframe, but as you found their engineer is quoted on the Cumulus website as recommending 30cm minimum diameter... I would ask them and refer them to the aircraft manufacturer's TN spec of 13cm and see if they think it's alright.

Just for good measure the AC43-13 says that a transponder ground plane should be 12 inches diameter minimum.

Isn't t it nice to get contradictory information from the authorities? Confusion is fun!

I'm not sure whose instructions are considered the final authority under the FAA. Perhaps it's the aircraft manufacturer's as they give your A&P and avionics shop the only instructions specifically about your glider to use to authorize the installation. It seems unlikely that Schleicher's instructions would result in unacceptable performance though given how many gliders flying in EASA airspace must have had transponders installed in accordance with them by now.

The L2 antenna does seem convenient but it sounds like the original poster requires a TSO'd antenna. The L2 isn't so far as I know.