For the Nikon D3x and D3s and D800/D800E, consider the Nikon DK-17M magnifying eyepiece. Certain other models have equivalents.
A number of users have written with concerns about how to make manual (by eye) focus more accurate. The DK-17M raises the odds in your favor.
When working with Live View, the Zacuto is what I use, by holding it against the LCD (I don’t use the bracket).