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