I use Skydemon an a cheap Chinese GPS under a greenhouse canopy (AT-3) and it's difficult to read directly in bright sunlight. Ok in normal conditions, but I can't read it directly on those rare hot summer days.
I have it positioned just to my left under and to the rear of the vent so it's about 12" away from my eyeballs. It's still useable, just means that I have to shade it with my hands to read it but it's not really a problem as I only glance at it now and again to check we agree I'm on track.
There are screens available though a bit of cardboard secured with Bogie tape would probably do the trick.
I would imagine most devices would be the same.