Hi Tony.
I have 2-tier rack like you described. I took a standard wood-grain board that you find in the shelving units section of a lumber store, Home Depot, or Menards. Its 4ft long, 12" deep, about 3/8" thick. I placed it accross the top tier. It don't look bad. Ive seen these boards available in white and black too. I only play as a hobby at home and this was cheap and functional. A hole or two could be drilled thru the board to run a small velcro strap through to secure your PC.
If you want to spend more money, you'll probably find something useful in the catalog of a manufacturer of DJ equipment or something similar.
I remember seeing a multi-tier rack at a Guitar Center or Sam Ash that had a tier designed to hold accessories. Don't remember the brand name but maybe someone out there does. I'll see if I can find it.