I like it! I never thought of putting one under the cover in the rear and usually just carry a Mini-14 in a gun case covered with a blanket. It would be nice to make a foam cutout to fit the space, the gun and a few spare mags. While the space I have is too small for a Mini-14, I have a couple of AR pistols and one in particular with A2 sights would work nicely.
A tip for people who hide arms like this. Please put a note with your will or some other paperwork that your estate executor will find. We went through this last spring and only found the hidden pistol in the truck because the empty pistol box was still in his safe (combo was with the will).