For the last ten years or so, I've used a Velbon focusing rail for my 35mm Nikon equipment and my Bronica EC-TL system. Works very smoothly. It allows you to move the camera very precisely forward or backward, left or right. Very sturdy.

Enter 'velbon focusing rail' on Google and you'll see it. The current version is called a Velbon Super Magnesium Slider. Looks very similar to the one I have but I'm sure there are improvements over my 10 year old one.