I bought a set of Cambridge Soundworks PC speakers with four speakers and a subwoofer, although I'm only using left+right+woofer in my cab. I used the removable plastic "feet" of the smaller left/right speakers and screwed them into the sides of the cabinet, just inside of the speaker panel. Because of these "feet" I can remove/replace these speakers easilly if I want to. The subwoofer was mounted on the back panel at the height of the speaker panel using shelf brackets.
At first I was worried that these speakers inside my massive 3/4" MDF cabinet would create a lot of resonance but I was pleasantly surprised when I noticed that the cab actually enhanced, not distorted, the bass of the subwoofer. Although I suppose you could get even better sound with real HiFi-speakers I'm very pleased with the way my cab sounds and I often find myself using my cab as a MP3/CD player instead of my mini stereo.
An added plus of the speakers I have is of course the remote volume control which can easilly be mounted outside the cab.