I just installed Hoodman’s HoodEYE eye piece on my Nikon D3.
I used to use similar eyepieces back in the day on my medium format cameras. The HoodEYE easily installs in place of your stock eyepiece and helps block out light from the side that might reduce your ability to see well in the viewfinder.
I installed mine in about 50 seconds. Hoodman makes the HoodEYE for all Canon and Nikon DSLRs.
The HoodEYE eyecup rotates to accommodate both right and left eyed shooters.
The cost is $29.99 from Hoodman.