I know you have it already made and painted, but I see tonneaus on craigslist by the handful every time I check. Pretty cheap too. I got one for $100. I see 'trifold' models too, which might allow a partial tonneau effect to be achieved.
As for their effectiveness relating to fuel efficiency; I must admit that my experience has been that it has little to no effect. I do a fair bit of highway driving, which is where gains should be seen, but I'm not seeing them. The best thing I can say about my tonneau experience is that it keeps all of the crap in the bed from blowing into the cab when I open the sliding rear window.
