I have a Sundowner 2-Up seat on my FXDWG. Here is an alternative that I am considering.
Cruiser Bags
These bags are the strongest things I have ever seen. The walls of the plastic cylinder have to be something like 3/8" thick. You could almost support the bike with one of these things. You could jump up and down on this thing and not make a dent into it. In the photo you can't see the integrated padded back rest that the girl is sitting back against. It is cool as hell. Check the website link to see the back rest. With proper sizing and their seat rest, you get a perfect rigid back rest plus a ton of storage for a long trip.
I talked to the owner of this company and asked him a lot of questions at a cycle show. These folks know what's going on. The product isn't cheap but it is top quality and should last just about forever.
