If you remove the entire undertray (something like 14 bolts?) on the underside, which is common for various maintenance, it opens up a gap to reach in there from underneath, between the back bottom edge of the bumper and the metal rail that the radiator sits over. It's generally enough to reach into the front of the coolers if you want to install or remove a plate, but it might be tight access for doing the initial setup of anything complicated.
As for aluminum, check the Hardware section at Lowe's or Home Depot or whatever. They almost always have a small section of raw materials in the hardware aisle: steel bars and angle iron and sheets and such. Usually among the (quite limited) selection there will be some very thin Aluminum sheets (I think they're .025" thick?), something like 12x24 inches in size, which is perfect for cutting up for something like this - they're very light, and you don't really need more thickness/rigidity if it's just a cover.
(And if they don't have aluminum, or you ever need a real selection of materials:
http://mcmaster.com has *everything* and they ship fast with good customer service).