Since I'm right in the middle of building racks....
Assuming it's a standard 19" rackmount... Like a Smackie or a Mix Wizard... the inside width dimension of your plywood carcass is 19.25".
Rack Units are 1.75" each... that will determine your depth... but I would imagine that the mixer is probably 12 units, or there abouts. Measure it to be sure... then gustimate the extra off the back side to make it easy to make your connections. (about 6" extra)
Make a complete box first. Then take a saw and saw a lid off at about a 15 degree angle.
Add rack rail to the bottom and some latches to the lid.
Prolly half a day's work.