I just roll my pants up in any weather, but I have been accused of having an unusual tolerance for cold. What about picking up a pair of cycling leg warmers (the sort that the lycra crowd* wear along with spandex shorts for in-between temperature days) and slipping one on your left leg under the pants? Then roll up the pants. Or knee-high socks, if they still make those in sizes other than schoolgirl.

* no mockery implied; I am part of the lycra crowd at times! Just not when running errands.