Prusa i3 Plus - Improved rambo mini cover.

Several improvements to the Prusa i3 Plus rambo mini cover.

* The hole in the middle is a real pain to screw in, so used the lower hole in the frame.
* The top hole has little clearance for an allen key so increased the clearance.
* Made the cable entry points stronger so the main wall thickness can be reduced. (which reduces print time)
* Combined with the back plate using a one layer plastic hinge which saves two screws.