I say go ahead and grab a board from a store or take the one out of your kitchen if you can get away with it
If other people have reported increased mileage then you really won't have much to lose by giving it a shot. If nothing else the leftover part can be saved and put to use in your kitchen like it is supposed to be or used for fabricating other parts. The plastic is pretty easy to work with and if you warm it up it seems like it could be formed easily.
It took me probably 30 minutes to make the first one shown in the pictures. I Could probably make one in 5 minutes with a drill press. If I had a hole saw assortment and some routing bits to go along with it.
I am probably going to make one to go in my Astro van sometime or another as well. It will probably give me a more accurate number to show how much of an increase there is from it. The Metro is getting some other stuff done at the same time so I am hoping I can tell what it gives me but it might take a few months to sort out how much it helps that car.