As I suspected: a recent news article reported the box was identified as an “AN/DMQ-1 gondola” balloon reconnaissance payload from the 1950s, part of an operation called “Project Genetrix “,
intended to collect information about the USSR and China. The news report, referencing information and photos from the “Military Communications and Electronics Museum” in Kingston, Ontario and “declassified documents on the CIA website” also says that there were 516 balloons sent up, with only 34 recovered (I guess that’s 35 now).