EDIT 4/7/2016: V2.
3 major changes:
(1) substantially decreased the width of the holder/bracket. Regain some print area.
(2) added slots for nuts inside the bracket (M3 size). No need for a wrench!
(3) added zip-tie slots on the top to ensure it doesn't fall off the x-axis rods.
Also modified the "clips" to fit better, modified some of the dimensions to fit better.
From original post/v1:
I whipped this together based on other x-axis endstop holders/mounts/etc I've seen around for other printers. This one I made to work with my modified ThreeUp. The other x-axis endstop designs I've seen typically depending on still having the gantry or other "factory" parts I've since swapped out.
Note: the curved ends intended to snap on to the steel rods both broke a little when I tried to snap it into place. However, mine still snaps into place, even though a little bit broke off. So, if that happens to you don't panic, It will likely still function.
I designed in the holes at the top and bottom for bolts, zip ties, or whatever else you may need to ensure it stays in place. Fortunately mine doesn't need additional support at this time, but it is there if you need it.
Supports set at 30 degrees; 1.75mm filament; max speed 50; 196C/194C 1st layer/other layers; 6 perimeter layers; took about 50 minutes.