The pressure differential should be minimal to get those 5 more degrees of
C.  Just put a lid on the pot and hold it down that should get you up there.
At my home in Wyoming at 7500 ft water boils at 197 deg F about 15 deg below
normal and 221 is only 9 deg above normal.  The pressure differential is
only a few pounds a lid and a few pounds of pressure should do it.  Let us
know if it works

Jere
90 GL
88 GL