The hard part is done, you already have the supporting brackets in place. I had to fabricate my supporting brackets before I could address the case attachments. I use a bracket that "captures" the case with a padlock in the case latch:
The cases are Pelican 1510M which are a little bigger than I was looking for. I had decided on the Pelican 1500 cases when I found these 1510's on the local craigslist for $25 each. I almost bought four at that price! Fully loaded with tools they can make the front end light so careful how you load them.
I used "U" shaped hooks on the supporting brackets and tabs with standoffs on the back of the case. The standoffs on the back of the case nestle down in the "U" shaped hooks on the brackets. The "capture" bracket hooks to the support bracket, lays across the top of the case, and a tab on the end of it has a hole for the padlock that is used on the case latch firmly securing the case to the support bracket. Remove the padlock from the case latch and the "capture" bracket swings out of the way for the cases to be lifted from their support like luggage.
EZPZ
