I'll agree with Darryl that the rear fuselage / tail cone is a good place to mount an L2 antenna – provided that you can reach the area
AND get an adequate coax there. See

https://www.timesmicrowave.com for a coax calculator.

Do make very sure that nothing will come loose and interfere with the elevator pushrod, or anything else.

Darryl and I disagree on mounting an L2 antenna in the forward fuselage.

That said, my installation forward of the rudder pedals in my previous ASW-20 passed the transponder test and ATC had no complaints.

To shape the balsa, stick 80 grit sandpaper on the site with double sided tape and sand away.