Cameron wrote an answer about Light & Motion Vis 180 Tail Light on April 14, 2014
I get pretty close to maxing out the strap on a 31.6 mm seat post, it only leaves me a couple of loops to work with. You can mount the light on the seat stays as well, just turn it upside down if you want the light perpendicular to the ground. Or like Don said, mount it to a saddle bag with a loop. You can also clip it to a back pack or a jersey pocket (not reccomended). You might also consider the Vis 360. It adds some weight to your helmet, but is a good way to see and be seen. It is easy enough to take off the helmet and store in your jersey pocket when you don't want to use it.