I haven't seen any pics of MKS Gen L builds, so I thought I would share mine.
In my opinion, the MKS Gen L provides the path of least resistance if you want to build an OnStep controller. The Gen L is an all-in-one 3D printer controller that includes all the hardware that you need except an optional wireless access point, which can be easily added. It uses the Ramps 1.4 config file in the OnStep firmware. I found no bugs in the firmware. Kudos to Howard and everyone else involved in the development.
I built mine into an old police scanner chassis. As a bonus, the scanner had LEDs on the front panel to show which frequency channel the scanner was monitoring. I retained the LEDs and connected them, via resistors, to provide status on power and stepper activity. They aren't necessary, but they are handy for troubleshooting.
I labeled my pic of the completed controller to show where the connections are made. This will will be helpful, but it won't be enough information to build one. You will still need to carefully read and understand the available OnStep documentation. All of the information that you need is provided, but you will have to do some detective work because it is in various locations and is mixed in with info on other controllers that OnStep also supports.
I am using it to control a large 1960s GEM carrying a 16in Skywatcher Newt. The mount came complete with it's original 7watt synchronous drive motors, but no controller. After disassembling everything and cleaning out the dried grease I cobbled together an ST4 controller using relays and vintage drive correctors. This worked OK, but when I heard of the OnStep Project I knew this was the way to go.
Please don't laugh too hard at the 4 wheel cart and the scope cradle. They were built out of scrap that I had laying around (2x10s, old farm equipment, etc). The whole monstrosity weighs 1200lbs. I roll it in and out of my hangar. This is intended to let me assess the capabilities of this mount/telescope so that I can decide how much I want to invest in a more permanent solution (observatory).