The regulations seem to be deliberately written to prohibit exactly
what you are wishing to do. Seems like no "freelance" avionics guys are
to be allowed in business. The FAA only wants certificated A&P
mechanics and certificated repair stations working on certificated
airplanes, and lately there seems to be some kind of subtle pressure to
begin elimination of the independent A&P mechanic too and try to make
all airplane work required to be done at certificated repair stations
with lots of extra bureaucratic oversight and scrutiny.