I noticed when filming outside with my Atomos Ninja 2 that, because the screen was so reflective, it was hard to see what was happening on the monitor.
I looked on the internet for solutions, but I couldn't find anything of use, so I decided to model and print my own.
To print one for yourself, download the STLs and import them into your print controller.
I modelled this using SolidWorks 2013, so to check out the source and make modifications you'll need to download all the
I printed this on an HP uPrint SE Plus in white ABS, so that I could later spray paint it black.
Cut the eyecup off a cheap eBay viewfinder (like this) using a mitre saw, then hot-glue it onto the viewfinder body.
Get a GoPro mounting bolt, also from eBay, and it'll fit together nicely.