After a lot of searching, I found
this clip. Totally worthless to me since it can rotate. I searched McMaster Carr and Grainger under "clip", "mount", "fastener", "quick release", "slide", "rail", and "bayonet". Nothing quite right, but a couple sheet metal things that aren't tight enough to do the job.
Anyhow, I think I could just leave a couple pan head bolts exposed (slightly above the surface and painted black), and then make a plate shaped like an E, but rotated to have the fingers of the E pointing down. The spaces in the E would slide under the exposed bolt heads. Pretty easy to whip up. I hope that's at all clear. I do have a dovetail cutter, but that'd require thicker stock and more work.
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Main Entry: co de pen dence - see codependency
co de pen den cy
Pronunciation: \kō-di-ˈpen-dən(t)-sē\
Function: noun
Date: 1979
: a psychological condition or a relationship in which a person is controlled or manipulated by another who is affected with a pathological condition (as an addiction to alcohol or heroin) ; broadly : dependence on the needs of or control by another