This is how I mounted my fire extinguisher and first-aid kit. The audio wires are hidden under the carpeting and fastened down, the CD changer bracket serves as a base, and I fashioned a mounting plate to hold the fire extinguisher.
The spare is in my basement, although I did use it recently (long story).
The question now is where to place the warning triangle and toolkit roll? Carrying the former is mandated by law, the later does come in useful. I like a clean look.
-Paul