What is the best way to weight the sheet down?
It is best to apply an even pressure over the whole surface of the clamp to prevent the sheet flapping and air getting in. Touching tyres can be used on unroofed clamps and either tyres or hay/straw bales on roofed clamps. A few tyres or bales scattered over the top is not sufficient.
Alternatively, use a heavy duty woven tarpaulin with gravel bags around the edges and seams. Although more expensive to buy they can be reused many times.
Note: 10 cm of surface waste actually arises from a much deeper layer of previously good silage. DM losses in the surface waste can be as high as 65%.