I installed a "custom" bed rug in my truck and it is awesome! It's really simple to do, so heres how...
First, go to Lowes and get a few rolls of indoor/outdoor carpet. It is cut in 6'x8' pieces, so it took me three rolls to do the entire inside of mine, including the underside of the tonneau cover.
Also, get some kind of silicone equivilent to Liquid Nails. Some stuff called "Beats the Nail" was used on mine. This stuff will glue it on and never come off.
Now measure out your demensions and cut the carpet to desired length and width. Lay it down where you want it, make necessary cuts to make it fit and cover the back side of it in glue. Lay it down on the metal and hold for about 10-15 seconds and let dry.
After you have it all glued down, stand back and admire your work...