I've only ever taken the cover plate off my car to reveal the underside of the tank, but when i did, there was a triangular section of squashed foam stuff on the cover plate. So i guess a triangular piece goes there. I remember seeing pictures else where, and i'm sure there was two long rectangular pieces, one for each vertical long length down the sides of the tank, and then a shorter one for the back vertical wall of the tank too. I think also, my tank has some "L" shaped foam pieces positioned on the top long edges where you can see a couple when you remove the access panel to see the fuel sender and pump etc.
In fact, i've just looked, it shows you here:
http://store.delorean.com/c-274-2-1-0-t ... oretc.aspx
When I had my cover plate off, I renewed the old triangular pad with some sound insulation foam you would use to pad the inside of the your PC case to lessen the noise. Seems just fine for a bit of padding, and better yet, didn't cost me anything.
Was like this sort of stuff, cut to shape with scissors. Had some already, which I never used:
http://www.mnpctech.3dpixelnet.com/pict ... iew9z4.jpg
(Although I think my stuff looked a bitter more rigged than this. This stuff looks quite soft and squashy. Mine has a bit more sponginess to it.)