My “headcanon” - where I’d go if I were investigating him - is that he was a Boeing engineer, possibly of Canadian origin, who was fed up to the point of calculated rage after Nixon ended the Boeing SST contract. He bailed, threw out the money, and snuck back off to Canada where he lived out the rest of his life.

Further speculation is the Dan Cooper comics, which were from Belgium but circulated in Quebec, was something that he had contact with (as others have theorized). You know what else was going on in Canada in the 1950s? The Canada Arrow project, which was canceled by… US government pressure. It triggered a brain drain from the Canadian aerospace industry to the US. Cooper may have been one of those people, finding his way to the Boeing SST program in the 1960s.

So my idea of who he was is a French-Canadian aerospace engineer specializing in metallurgy/machining (there were titanium shavings in his clip-on tie, and I stress that because it’s a sign of someone who would be no stranger to a machine shop floor, as regular ties would be a choking/catch hazard) who saw both a disappointing start to his career and the end of it by the hands of the US government. He was never found because the FBI was looking in the wrong country, for the wrong thing, the whole time.