I've got some experience.
got a Navon gps navigation device and put it on a Hama holder inside a case (also by Hama) - there's holders, too, but not for Navon, so had to use case.
all in all cost me like 35 euros (holder and case) and works just fine.
am using my iphone's headphones connected to the GPS and it works perfectly. you need perhaps 15 minutes to understand how the navigation is navigating, then it's easy. sometimes you check the screen, but that's really just a secondary helper.

there's two Hama holders: http://www.hama.de/portal/articleId*138455/action*2563
and http://www.hama.de/portal/articleId*154354/action*2563
I got the first one, becuase the other one would cover too much of the dials. anyway, as mentioned, you're not gonna watch the screen too much, so that's it.

btw, if your helmet enables bluetooth thingies, a navigation with bluetooth connectivity might be better than cable headphones (it's a nuissance when mounting/dismounting the bike, else it's ok. when choosing navigation, it's worth to check how bright the screen is (said other forums, but I don't really know the difference, to be honest. if it's too sunny, you're not gonna see anything anyway).

I bought the hama holder and case in vienna in Saturn. there's some web shops in slovakia who carry it, I think. try googling