cutting mdf board

Use a skill saw and lock your angle you want.

Yep sure can mount your amps to the back of the box, but it just as easy to make a amp rack out of the extra mdf and you can make it look a lot nicer since you can run all the wires behind the amp board for a cleaner look.

Here's an angle calculator that I often refer to when cutting wedge shapes.  It will give you the angle of each cut.  Best way is to set the angle on a table saw, but you can get real close by marking the cut angle using a protractor and setting up the piece solidly so that you can run a circular saw blade through it.

The best way to achieve a secure joint is to add an additional strip of MDF on the inside of the angled cut end.  This will add greatly to the glued area.  Make the angled cut on the strip by first assembling (before glueing) the angled cut to the flat piece.  Copy the angle of the inside (there's a tool for copying angles, or you can improvise) and slice the angle off the bracing strip.  Glue it all up when you've found it to fit.