The aluminum Flip Follow Focus from Proaim can be mounted on any 15mm LWS rod system and allows you to make smooth focus adjustments. The follow focus comes with a 0.8 MOD drive gear and an adjustable 0.8 MOD lens drive belt. The unit snaps onto the 15mm rods, so you don't have to remove other rod-mounted accessories to add it to your rig. It also features side-to-side adjustment, allowing it to work with different-diameter lenses.

A magnetic white marking disk is included for marking stop points, and integrated A/B hard stops provide physical stop points. A hinge connects the follow focus to the rod clamp and it can tilt up and down to easily adjust to wider diameters. The gear box can also be "flipped" to use it on the front or rear of the lens. A speed crank is included, and a focus whip docket on the follow focus allows you to use a separately available whip.
Can be mounted onto any 15mm LWS rod systemDrive gear can be mounted in front or rear positionSlides and tilts to fit different lens sizesSmooth fluid movements for easy focus adjustmentsIntegrated hard stops for precision focusingRugged aluminum construction for durabilitySpeed crank and adjustable gear belt included

review from bhphotovideo;
While I did not buy this, I worked with this or it's immediate predecessor pulling focus on a scene for a movie--rigged on a Canon C500 with other accessories. I have been a professional camera assistant for years and I was very impressed with the quality and functionality and amazed when I heard the price. Just used one day, but impressed enough to look it up and write this review. The hard stops helped overcome an issue on our still photo lens which has a clutch that allows the lens to continue to spin if you pass infinity or minimum focus. The stops can be placed at the ends of the range, thus if you have set focus marks you won't risk focusing past the range and having the clutch slip and throw off your marks. Also the stops could be used if you need to hit a fixed mark quickly without going past. Overall seemed precise and sturdy without slop or backlash.