Schleicher TN 2008_TM02_E_HB which is about adding equipment says that unless an instrument weighs more than 1Kg it's OK to secure it with just the four instrument panel screws. A Becker or Dittel radio would weigh less than this.

I'm currently finding out whether my old FSG-40 is still serviceable. It's mounted at the very bottom of the panel which works fine. Problem is that there's not enough vertical space in that location to fit a modern 57mm Dittel so if I need a new radio I may end up mounting it at the far left where the mechanical vario is now and leaving the empty FSG-40 bracket in place. That bracket is the only structure behind the panel so I used it to mount the new Powerflarm core this past spring.