See your idea here, it's a good one to make a lumber support rack, I just used the cab a few times on my 2000 Sierra and have the dents to prove it. Maybe look for a 2 X 6 or 2 X 8 thats cupped but not twisted to match the roof line a bit, don't angle it down much, too much angle won't help a bit, go maybe 2° max. I read somewhere that raising the height of the bed helps MPGs too. It may look like crap but you may wanna make your cap with 2x4s edge wise on top of the rails. Also, covering the back half of the bed gets you like 80% of the FE gain from having a full bed cover.
Here's a crude pic of what I'm thinkin
