Outer box is a 15" toolbox, less than $6 at fleet farm.
Inside:
-12Ah SLA battery, UB12120, approx $25 on amazon/ebay/etc.
-Radius M100 radio (have a pile of them laying around)
-TinyTrak3+ (Byonics, $33)
-SkyLab SKM53 GPS module (ebay, ~$24)
Built the TinyTrak3+ with a small modification to allow higher audio output - replace R5 with a piece of wire.
Simply attach a small mag-mount antenna to the radio, toss in the back seat, and it's set to go.
Final result:
