I have the happy trails racks with the Wolfman Expedition bags... I wanted to keep my bags on my bike all the time. The problem with the bags is they have no form to them without anything in them. So I wanted to add something to inside of them to add some form to the bags. I chose a 5 gallon plastic trash can to do the job. Description and pics below. So far I am happy with the set up!
I cut the top off and cut a slit down the side so I could fit them inside the bags.
I left trash cans inside the bags for a few days and realized the sharp edges were somewhat showing through on the bags so I added duck tape to the edges to get rid of the sharp edges.