Hey Mike, hey Tom, hey Alden, hey all,
Onefinity CNC uses Stall Homing. Stall homing means you do not have a sensor or something that indicates that the gantry is at the end, but instead the controller realizes the stepper motor suddenly generates no more power, deduces from this that it obviously does move no more, and states “it is at home”.
This corresponds to the philosophy of the Onefinity CNC to allow customers a low-threshold entry into the CNC, which among other things happens through a low price that is achieved by saving on everything that is possible (mind you, without affecting the important things making the Onefinity CNC a very interesting machine). One drawback of stall homing is, as some people reported, wood dust tends to adhere on the rails and to accumulate at home position, thereby changing stall homing position with time. Generally you can do a few things to improve stall homing, one thing Bill @Machinist suggested and Tom @TMToronto already mentioned above, is mounting a bolt as alternative stop surface:
But in principle, you can turn your back on this cheap stall homing and retrofit what is used in better equipped and also in industrial machines: Proximity sensors as Limit switches. The good thing is, the Buildbotics Controller, which is the base of the Onefinity Controller, supports them. You can wire them to the 25-pin I/O Port and enable them in the Motors Tab and of course their status will show up in the Indicators Tab (hope it’s okay to quote your pics @TMToronto Tom, even if you’re using another controller!):
Some people also use microswitches but I would always prefer using proximity switches.
The problem to solve anyway is how to attach them. As far as I know, everybody used 3D-printed parts, and I will do it this way too. An alternative would be milling them out of wood using a CNC! 
Further reading