This would be a soft plastic that slips over the unused slots in a mother board.
You could make them a variety of colors, glow in the dark, etc. . .
I have had this idea for years, even went so far as to try and find someone to manufacture a few prototypes. Manufacturers being wise, they wouldn't do it for a piece of possible sales. I even offers 80% just because I wanted to see if it would take off!
I figured I would charge 3 bucks a pop.
Now some people will say "Who would want that?" and to those people I suggest they look at all the money people spend on the bling of their rig.