I used some science-project knobs, grade 8 bolts, big ol' 2" neoprene-lined fender washers, regular washers, and some carved-up hockey pucks. End result: panniers I may remove at my whim, in a minute or two. Details dwell here.
The diagonal braces tie in at the seat pan, and any excess deflection is damped out nicely from both the high-density polypropylene of the milk crate (high-frequency, low amplitude oscillations) and by the elastic of the bungee cords holding said milk crate (low frequency, high amplitude stuff). The velcro at the cylinder guard/frame downtube also handles helps to tie things together.
It's called engineering. Some call it "art" or "shade tree improvisational mechanics".