If all you wanted to do was to add a very basic Follow Focus to your camera, you still end up needing a very basic set of 15mm rails and baseplate. Although there are literally dozens of baseplates available online, here's a new PVGear 3 Way Baseplate that adds a horizontal 90 degree rod clamp integrated into baseplate. This saves from having to purchase an extra part as well as save space on your rails.
Example: Sony A7s with Atomos Shogun 4K HDMI Recorder
Here's a quick video on how I assembled a few pieces to create a super short, lightweight, and functional rig for my Panasonic GH4.
Instead of a cage system where you would mount all of the accessories onto the camera body, the horizontal rod allows you to add a monitor or even portable audio recorder alongside your camera. It's quick to break down and setup for travel. With a plate under the base you can add your favorite quick release to go direct to tripod mount. Like many affordable baseplate systems for rails, this is a basic setup that does not offer rail height adjustments which can be handy when using cheap matteboxes that don't offer up/down alignment. Still if you don't use a mattebox very often (as I don't), this is a handy piece of kit.
Example: GH4 with F&V SpectraHD Monitor and PNC Hybrid Should Pad
The top plate can be used to mount your camera directly or add a QR base system like the Manfrotto 577. You can find the new 3-Way Baseplate with or without Carbon Fiber Rods available at https://pvgear.com/products/3-way-15mm-rod-dslr-base-plate-mounting-system.
PVGear 3-Way Baseplate for 15mm Rods
PNCGear Hybrid Folding Chest / Shoulder Pad
Manfrotto 577 Rapid Connect w/ Sliding Mounting Plate (501PL)