A few more options - Those 3M command strips come in handy for plastic cased power bricks, just floor mount them as they still get hot enough to soften up the adhesive. Use Cable clips for wires. A computer can be screwed right to the inside wall of your cab (If you dont care about getting screw holes in it). I've screwed strapping material directly to the wall and floor of cabs before as well. Oh, and L-Brackets are also your friend. They are great for screwing in bulky things, and you can also thread zip strips through them.
If you mount a sub inside your cabinet, I'd recommend putting down rubber feet or something to reduce the vibration and also keep it apart from your computer. Just as a precaution - Vibrations and magnetism can hurt a hard drive, and a sub makes both.