I've been here before but I can't remember what I eventually found that was exactly what I wanted (but not called a tank connector).
I need some 'flanged tank connectors' but much smaller than is found by simply searching for 'flanged tank connectors'. I want to connect something like 10mm tube to a flat plate so that there is no projection at all the side away from the connection. A flanged tank connector provides exactly what I want.
I seem to remember asking here about it a year or more ago, and I found what I wanted then, but I can't remember what it turned out to be called. I think it's some sort of furniture fitting or similar.
Any ideas anyone to help out my aging brain?