OK, DH and I went to the hardware store to buy some sheetrock. We planned ahead, had a blanket to protect the roof of the van, bungie cords to hold everything down, etc. He stacked the sheets, corded everything down tight, and we started to drive home. One of the bungies failed about a mile down the road, the cord sprang back and broke both the left and right rear windows in the van, and then the sheetrock went sailing behind us, crashing on the road. We both thought we'd been shot!
Looked at each other, looked in the back of the van (full of glass), and then looked onto the road behind us, hoping that no one had been driving close to us. Phew....no crashed car behind us, just powdered sheetrock! Needless to say, we now only carry sheetrock inside our bigger van (or have it delivered).