The Nikon D750 is an amazing bit of kit, my only complaint is that the eye cup falls off far too easily.
Here is a video of how I fixed mine.
All you need is some black gaffa (duct) tape and you are in business 🙂
I hope that helps all the d750 users out there, it certainly stopped my frustrations.
If you are a YouTube user, head over to my @paulkporter channel and subscribe, I will be posting more videos of hints and tips on there.
And remember to follow my journey 🙂