What should I use as a base for my wall mounted PC case?
Your circuitry and peripheral devices could be mounted on a piece of plywood. Such plywood could be pre-painted if you wished to get a desired color effect. For a more unique effect you could cover the plywood with a layer of sheet cooper similar to that used by roofers. This could look very cool of it was all buffed out to an exquisite shine and sealed with a coating to keep it from tarnishing. A clear lacquer finish may be just the ticket.