(yet another) C270 Camera Arm

I came across one_2_three's design for a camera mount and it ticked almost all the boxes I wanted:

* Mounts directly to the Y axis frame, solidly held in place with 2 bolts
* minimal height
* minimal depth (so it'll fit in my Lack cabinet with the front on

But it lacked a couple things I needed:
* stronger arm to avoid sagging (that reportedly happens with a lot of mounts)
* screw in mount (borrowed from jacobtc's snap-on mount - which I'm currently using).

I also tapered the screw holes in the arm to match the M5 screws I have on hand