I was not aware that Canadian amateur-built aircraft need a special flight authorization from the FAA to operate in the US. Are people actually getting these? It says in one place that they normally don't expire and in another place that they're issued for a maximum 180 days.
https://www.faa.gov/sites/faa.gov/files/aircraft/gen_av/ultralights/sfa/amateur.pdf
https://www.faa.gov/sites/faa.gov/files/aircraft/gen_av/ultralights/sfa/amateur.pdf