XFCE: Set default extend-to-direction?

[Paul Johnson]

I have a laptop that I frequently use with an external monitor, but on the
lefthand side.  Now by default, XFCE assumes that I want the monitor to be
on the right, and the top of the laptop to be even with the top of the
external monitor.  I would love it if there's a way to get it to assume I
want the external monitor on the left with the laptop bottom-justified to
that instead.  What's the best way to accomplish this?