I have a Navman 7100. It too hada aslippery Cover. What I did was stick a couple of layers of Ducktape on the inside and it sticks very nicely and tightly.
As for build quality. It has got very wet with sea water and still works. It comes with all the extras you could want, Diferent cables and stands etc.
Basically it does the job at a good price.
The only thing i have noticed is that the little spring on the DSC Emergency flap has rusted slightly.So what i have done to this is cover it in Petroleum Jelly.
It works well with the Garmin GPS although the last couple of digits on the Lon/Lat are different. I don't know what this equates to in actual distance but I shoul imagine its only metres.
Let me know how you get on.
The next thing I am going to get for it is a remote speaker because I am getting old and have become partially deaf. Or is it because I've got a Merc 2 stroke !!"!