I use my cheap Chinese sat nav with the following simple mod:
Shroud for Skydemon GPS
'I use Skydemon on a cheap Chinese 7" GPS in a Jodel - a lot of canopy and a lot of light. What I did was to put two velcro tabs on the gps top corners. These tabs match up to two velcro tabs on the underside of rectangular 1lt ice cream tub (Kelly's Cornish Clotted Ice Cream - sure other brands will work just as well ) which I've cut out to match the the screen size.
It works pretty well and the shield is quickly removable in flight.
I've also seen suggestions about removing the thin plastic sheet from the screen - not yet tried that but it would be quick to do'