It is a little late now, but it doesn't have to be a continuous mat if it is being used as a sound deadener. It works the same way your putting your hand on a cymbal to dampen the noise. Your whole hand won't cover the cymbal but just touching it will stop it vibrating. Sound is a vibration and can be dampened by contact with an absorbent mat.
If it is being used as a heat shield attaching it on the out side of the car where the foil can reflect the heat before it soaks through the car body would be the most efficient, but probably not practical (have no idea how good the adhesive is, or if you ever drive in rain or snow). As a heat shield you want continuous coverage, as has been demonstrated by our recent loss of the space shuttle that lost a four inch square tile.