I use lash straps (2) to tie down a milk crate to my rear rack. I am able to snug them up really tight and the crate is attached to the bike very securely. It took a few minutes to figure out the best way for me to attach the crate, but now that I have it figured out, I can attach and remove the crate pretty quickly. Lash straps are readily available at any camping store (REI, EMS etc.) that sells hardcore backpacking equipment and are inexpensive. I paid about $1.50 per strap.

When I first attach the crate to my bike, it change the center of balance so much that it flipped me over (ok, it flipped the bike over, but I was attached to the bike and waiting to cross a major state highway. Needless to say, I ended up on my tail-end beside the road with several honking/waving motorists )

Hope this helps