[_Ogre]

if your going with plywood it will have a seam in it as the bed is 50 wide
i'd make a frame and cut curved cross ribs out of 2x2 wood so it will shed water
place the seam on the plywood an one of the ribs and cover the whole lid with vinyl

or you could do the easy way and check your local truck accessory dealer
gm used that same bed for many many years
be prepared to pay, the topper on my 09 sierra cost me almost $3k