Hi Kevin,
I think you're okay using a US version. According to TechWalls, that cutout is the only difference.
[link|https://www.techwalls.com/iphone-12-pro-max-model-a2342-a2410-a2411-a2412-differences/|iPhone 12 Pro Max Model Number A2342, A2410, A2411, A2412 Differences - TechWalls|new_window=true]
[quote]The iPhone 12 Pro Max A2342 is available exclusively for the United States’ market. This is the most desirable model because it is the only one supporting the speedy 5G mmWave network with bands n260 and n261. Its physical appearance is slightly difference with a large side cutout for the new antenna.[/quote]
If that indeed is the only difference, it should work fine. Trying to use a Canadian version without the cutout would mean a US phone's mmWave probably wouldn't work well, but it should be a moot point going the other direction. The extra cutout won't hurt the other antennas' reception.