Dan,
Not sure what your intentions are for the tonneau, but for about $200 in materials, you can build a tough tonneau out of aluminum diamondplate. I made mine in 2004 and it is still in perfect shape. It can support as much weight as you can fit on it (i've had 4 people standing on it with no flex), and it fits just about flush and square with the bed rails, making it great for loading long items, like 2x4's, through the back window. Next week I'll be building a shed, and will load 16 2x6's, 50 2x4's, and 4 4x4's on the cover and through the back window. plus somehow I'll throw 8 sheets of plywood on top and 300lbs of bagged gravel in the truck bed.
Basically, I just bought the diamondplate pre-cut in 2 pieces, hinged them at the center, and hinged the forward plate to the top of the front bed-wall. Then used a filler piece to fill the center seam, and braced it in the center with the truck's jack. I've got 3 lashing points underneath that hold it snug to the rails. Close the tailgate, lock it, and nobody is getting in without a tough metal saw. When the full bed is needed, I have it hinged such that the back panel folds flat onto the forward panel, and then both can lean against the back of the cab, (with a towel between to protect the truck). I considered having it covered in LineX, but never got around to it. They quoted me $50 bucks to do it, and said it would bond perfectly and show every contour of the diamond plate.
Anyway, just an option in case you hadn't considered it.
