At least for the US why not keep one or more nuclear missile subs in the Great Lakes? Lake Superior alone covers 31,700 square miles to an average depth of 483 feet. It’s far to large an area to target with a first strike and well protected from non US/Canadian forces. Edit: A US/Canadian treaty demilitarized the Great Lakes. https://en.wikipedia.org/wiki/Rush–Bagot_Treaty
It’s a fun thought exercise though!