Go to a car audio shop. If they build their own sub boxes, they will have rolls of it in all different colors. Use a spray glue to coat the metal, mold the carpet over the trunk area, cut off excess. Exactly what you are looking for, I think.

 

here is a pic of my trunk. I know you said you didnt want to use wood, but its really not that heavy. I used 1/4" plywood that I bought from home depot. They carry 2' X 4' sheets for pretty cheap. I then cut them out and made two halfs out of each sheet. I bought black carpet from wal-mart and spray on glue from wal-mart as well. Molding carpet is not easy to do with any old carpet, might need to get some really thin stuff from online.

 

very true, never thought of it that way before though.. i dunno y

 

If done properly, the spray on bedliner looks really good. It also seems more appropriate (to me anyhow) than carpet in the trunk.