I never found a source to vacuum form the diffusion panel I mocked up earlier, so I came up with another idea. Rip 2x4's diagonally down their length, then use that "canted" piece to form a series of sloped ridges on a panel. In an amazing strke of luck, the local lumberyard (Alki Lumber) had a bunch of 10-foot "cant rip" 2x4's on hand and ready for me!