A simple solution to having an avionics master I have used is to have a
switch type circuit breaker that powers the Avionics bus.

I like the Potter & Brumfield W31 series try W31X2M1Gxx where xx is the
amperage desired.

It is possible for the breakers to fail depriving you of avionics. My
solution to this problem has been to install two such breaker switches and
to only turn on one at a time.

I like this better than relays which leave you with a single point of
failure that takes out all of the avionics.