Matt,
Old telephone books are good but do breakdown and become dusty.
Which might cover or effect sensors
Plates & bricks can rebound.
I find it best to use a Sand Bag , is best to put in a old sack if possible or several old pillow cases.
Can be filtered and old pellets recovered.
Light effect is normally the back sensors , normally cover with a sheet of paper, if giving error readings.
Blue paper seems to effect readings
BOB/R