Waveshare CM4-DISP-BASE-5A Case

I wanted a case for my Waveshare CM4-DISP-BASE-5A touchscreen that would present the screen at a better angle.  Carl-C's case is a great fit for the screen, but it's missing a back and legs.  This remix adds a back plate with reversible legs and ventilation for active cooling.  I also tweaked the original case body to add ventilation and slightly enlarge the SD card opening for easier access.  The back plate attaches to the CM4-DISP-VASE-5A standoffs using 4 M2.5 x 6mm screws, and the legs slide into rails on the back plate from either the top or the bottom.  One drawback to the back plate attachment is that it doesn't guarantee alignment of the back with the body.  If I have time, I may tweak the design to provide a method to ensure the back plate stays aligned with the body, but for now it gets the job done for me.