DSLR Solutions offers a simple and cheap follow focus solution that does not require expensive rails.
The DSLR Follow Focus system is simple, lightweight, requires no cage or rails and enables easy follow and rack focusing in a variety of everyday situations. It can be positioned for left or right-handed use and fits common lens diameters of 2.95-3.1 inches (circumferences of 9.25-9.75 inches) with custom sizes available by request. This unique system is pocket-sized, easy to put on and take off and suitable for the professional and casual user alike.
And here’s what you can achieve: