I don't know if your amp does, but some equipment like PC PSU's have certain points of the PCB attached to the metal chasis via screws. I guess the metal chasis is used as another conductor to complete the path, and for grounding purposes.
By this reason, and for improved mechanical resistance, I suggest you to use a metal box, with a window on the top and the front for example.
It's a lot easier to drill the window and glue the plexiglass piece, instead of making the whole box of plastic (making the cuts, mantaining the exact perpendicular angles while the glue dries, etc).
You have to remember that the box must have some means to open it again, for cleaning and maninteance purposes.
Also note that your friends will not see the bottom and the rear part of the box, no matter what material you choose, so don't spend too much money on that parts
Offtopic: This remembers me when I made the cardboard computer case hahaha