Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4848479.
A very simple but well fitting Z-max physical endstop for the Ender 3 and other printers that use a 2020 frame and v-rollers.
Recent Marlin uses #define MECHANICAL_GANTRY_CALIBRATION to provide a menu option and G-code to align dual-Z motors, it even works on two steppers working on one driver and is a port from prusa firmware. There's a second variant that requires two stepper drivers that uses the bed probe that can cause bed skew to drift as it levels from the bed and then you level the bed from the gantry and repeat...
You should have TMC steppers in UART mode to drop the current to use the G34 method, but I was disabling the steppers and winding both screws up by hand so the rollers hit the stops then homing. Before that I was resorting to two blocks under the gantry which meant finding what I'd done with them.
Anyway, the Ender 3 style frame needs these stops for all this to work and I made some that are a really good fit on the frame. The long tab is the top and this prints strongest as the stl is supplied on one side with some supports. It prints easy with the 3 tabs sticking up and no support but it's not as strong. I used cheap pla+ and it seems to be strong enough, but I printed it solid.
It's designed for M5x16 screws and you will have to tap a thread in the 2020 extrusion. Remove it from the frame before doing this as the screws securing it to the upright get in the way.
Here's a video of it in action, it explains better than I can. https://youtu.be/CWfAdz4vtpY