They're designed by me! This was a 3D print mockup. I ordered 10 CNC'd parts (6061-T651 aluminum + electroless nickel plating) for myself and to gift to friends. I've installed and ridden a few of them. Here's my thread: www.pinkbike.com/forum/listcomments/?threadid=249529

Yoshimura makes the ENDH adapter, but I don't know if they used a Boxxer hole pattern location or not (slight forward offset relative to steerer tube).
Hit-n-Run also designed one of their own.