My Magellan SporTrack designed for hiking/boating, etc., works just fine in the bottom/rear of the rear window of our 747-400s on either side. The cup holder makes a useful support, too.
Although not intended for aviation use, it agrees with the aircraft instruments to within 1 knot and 1 degree (at 500kts!). The only posssible confusion is that it displays 'heading' when it really means 'track'.
I find it works best if it already knows where it is when you get onboard. Even in the sweet spot the signals can be weak and it can take 20 minutes to work out where it is if you switch it on in the cruise.