I like a system with a quick release. I have a rack on the two bikes I use most often, and then I can just slide the trunk bag into which ever bike I'm using that day. It also allows you to easily slide the bag off and carry it into a store with you.
I personally like Topeak's MTX system, and I use the
MTX Trunkbag DXP. But Trek may have a competitive system as well (or other third party vendors). Topeak is probably the most well known for this as an aftermarket vendor.