Get the following:

4- 1/4x20, 3" long, machine screws or hex bolts, fully threaded
4- 1/4x20 wing nuts
8- 1/4" pan washers
4- 1/4" lock washers

Put one pan washer on each screw/bolt, put through bottom of the crate where they would just clear the sides of teh rack while the crate was sitting on the rack.

Put another pan washer, a lock washer, and wing nut on each screw/bolt, from underneath. Tighten fully.

You will have four screws/bolts sticking out of the bottom of the crate; each time you put the crate on the rack, arrange it right and strap that sucker down tight. The screws/bolts will keep it from sliding around.