I discovered that Cateye ViZ, Portland Design Works, and Planet Bike rack mount tailights all use the same mount form.
Planet Bike is cheapest to buy just the mount (6$) for using the same light on more than one bike.
When in town I use the USB ViZ450 and have a AAA battery powered PDW Danger Zone for off grid touring.

As an aside, Planet Bike also use a front headlight mount (they have more than one) that is the same as B+M and is only $6 allowing me to also switch my big B+M IQ headlight between bikes.