Hi Rumrunn,

The trunk bag has sturdy velcro straps at each of it's four corners along the bottom. It lashes down really tight. The bag is also stiff so it holds it's shape upright even if it's completely empty.

I Just started a cycling blog so I'll be doing reviews of my accessories, the Nashbar Deluxe Trunk Bag included. I'll send you a link when I've posted it.