Something like this?
http://www.thehotrodgirl.com/bicupho.html
I've seen these made by several people, and several different prices. This is just the first one I came across. I know $80 is a lot to swallow for such a simple idea, but it would make a nice finishing touch. I'm just not a big fan of plastic parts in a metal truck..... but that's only my 3 cents!