Have you consider the Pilot Aware Rosetta unit, can be stuck on the back window with wifi hooked up to a tablet running Skydemon, can see just about everything with audio alerts if connected, also can see gliders with flarm to 50 miles out. They are working on an upgrade that can give a bearing to mode C transponders as well.
https://pilotaware.com/rosetta/ Just a thought at under £300 for the unit and low noise 12volt power adapter.