Transponder antenna ground plane - Help!

While finalizing the installation of a TRIG TT21 transponder, I am running into less that consistent information regarding the ground plane spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod antenna inside the fiberglass fuselage.

The TRIG TT21 Manual has this to say about the dimension of the ground plane:
"[...] As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a SQUARE around 120mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700mm on each side."

From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."

Now, when I look at all of the Schleicher TNs, I see that they call out in most case a 130mm ROUND aluminum disk to be used in conjunction with the RAMI antenna mentioned above.

So what gives? Round or Square? Short of a very large plane, what dimension to use for the ground plane? The fact that the information available does not seem to converge on a given dimension or shape for a ground plane tells me that this is probably more than one "OK" answer.

Thanks,
B