I have the analogous attachments for my Minolta meter.

Groundglass readings tend to work best in the studio or at least indoors, where you can more easily shield the back from stray light. Outdoors where you're dealing with wind and the brightness of sunlight, it's easier just to take a spot reading and make the necessary calculations.

I often use a 5-degree finder and can usually manage to get the information I need from it, but there are times when a real 1-degree spot meter would be more useful.