Rich -
I feel your pain about location and wiring. My old Mooney doesn't have glareshield space and I often fly in other planes and wanted portability. Here's what I cobbled together to sit on the hat rack in back. It's two 10Ah AGM non-spillable batteries wired together for 12v output. There's an on/off switch and a voltmeter on the front side. The SkyRadar L (ADS-B) sits in the box with the batteries and the antenna (or antenna cable) sticks thru the top. There's a 12v receptacle on the inside for the SkyRadar, and an additional 12v outlet accessible from the outside for the iFly when I'm in another plane. A 12v charging port is underneath the switch for easy charging. It turns out the 2 batteries are overkill for powering just the ADS-B unit. A recent round trip to Dallas started with 12.8v and ended with 12.3v after 8+ flight hours. The footprint is about the same as a typical adult shoebox. See attached pic with top removed:
