I'm so glad you asked! I figured this out earlier this year and I've been so pleased with my dumb little system (which is very close to what murray missile posted above):

I have a rack with a flat spot on the top. I have drilled 2 holes in the flat spot and placed 2 bolts roughly 3" in length so that most of the thread is exposed upward. I used nylock nuts to secure the bolts. I also have 2 flat steel bars (they were the "ends" of a u-bolt setup) that I've drilled for the diameter of the bolts...and 2 wingnuts. I just drop my crate on the rack over the bolts, drop the bars over the bolts, and snug up the wingnuts. It's solid as a rock(!) and it only takes literally like 30 seconds to take on or off. It's far quicker than anything else I've ever seen and quite a bit more solid!