As a back-up, would the Garmin 96C handheld unit be sufficient?
I carry the 96 (the monochrome version) in my headset bag with a couple of spare cells just in case I need it. In retrospect the 96C is probably better value, with the colour screen. I've flown with it mounted on the yoke (mount supplied with the GPS) and it works well enough but in my present job I simply can't use it routinely.
I've used it a couple of times when caught in IMC to back up the 'traditional' radio nav and it gives great peace of mind that everything correlates. As with all these things, you need more than one source of differently-derived data for cross-checking purposes (i.e. dead-reckoning and radio aids). I think a hand-held like the 96C does provide a useful 3rd cross-check. Just don't EVER use it as a primary source of navigation!
Cheers,
TheOddOne