Anet A6 power supply cover with back switch

This is a remix of [Kingmiwok](https://www.thingiverse.com/Kingmiwok)'s [revised design](https://www.thingiverse.com/thing:2657572).

It appears to be compatible with both newer and older A6 **original** power supplies. This version is much sturdier than my [previous design](https://www.thingiverse.com/thing:2807723) and I much prefer to have the switch and socket on the back of the machine.

I've tightened the dimensions a bit (there was a bit too much play on the width; my power supply is