thanks for the advice, strapped to handlebars with webbing works great

Leaving for my first trip in about 6 weeks, a friend and I are going to trek 100 miles of the Ouachita Trail in Arkansas, allowing 4-5 days.  I think that I can fit almost everything in my pack as we are trying for minimal gear. what I am having trouble packing is my sleeping bag, in the compression sack it is about 12"x8"x8".  As of now I do not have seat bag and want to avoid panniers.  Any suggestions on a large seat bag that will not break the bank?