Mostly what I've seen is an insert that fills the bottom half of the double size factory hole. There is no cover or security. I would think it might be easier to just get some sheet metal and bend it into the box you want.
As far as the head unit install, most of these units come with a metal strap for securing the rear of the unit. It screws into the back and you are supposed to secure it to something under the dash. Given the PITA factor, I imagine most people, including me, simply forgo this step.
My latest unit, after water fried the last one, is a marine unit sans CD player, and it is only about 2/3 of the depth of a regular unit, so it fit in very nicely without using the rear support. I even used the frame from the old unit.