The default opto mounting brackets for the Prusa Mendel don't mount opto stops very well. Or at least not the ones I have, which have wires coming out the back. I like how Geoff has the orientation, but don't want to bend the leads on the opto stop. This allows the opto holder to be mounted flat on the bracket with screws and allows the leads to feed down without bending. It is also one piece to mount the stop.I used springs between the opto stop and bracket so I could make fine adjustments more easily. Trying to slide the stop up and down the rod a specific amount is a challenge.The printed end stop I know I saw someone else on Thingiverse make one just like it, but I couldn't find it again. If you let me know which it is, I can give them proper credit. It mounts to the end of the X smooth rod. If your filament isn't optically opaque, wrap it with foil. This will be a lot more resistant to damage caused by hitting it than strips of foil.I used M3 screws and nuts for mounting the stop. The springs were ones I got in an assortment pack so I don't know what size they are, other than they fit over the M3 screws nicely. I used an M4 nut and screw to hold the stop in place on the smooth rod.The holes on my opto stops are 19mm (on center) apart so the holes are spaced for that.I posted the SCAD file so you could easily make adjustments. The opto end is offset to one side because of the placement of my flags. Update, I uploaded the STL files. Sorry I forgot that at first.