These are the plates we use to push the beacons during our autonomous program in our 2016-2017 FTC Robotics season (Velocity Vortex). They attach to the Tetrix rack-and-pinion set. We found that the slanted angle increases the accuracy of the beacon pushers.
We printed the plates in ABS. To prevent warping, make sure the bed is very clean and you have at least a 5mm brim.