I have a large top case that fits my full face helmet with visor, but I have removed it due to what I have read on Helge Pedersen’s Globeriders tour page:
“We DO NOT recommend installation of a top case or trunk - in almost every "case", the rear carrier, rack, or trunk mounting bracket has fatigued, then fractured, and the top case has fallen off - in some instances, the riders didn't realize this until much later, and the top case was never recovered.”
The original owner of my GS had purchased a full set of Jesse bags and various accessories. There is a locking toolbox that goes underneath the top box, so it really sucks that I can’t use that. A cool Jesse accessory that I like is a plate that mounts instead of the rear seat, creating a large place to haul bigger things.
I know that there are different methods for mounting the top case, but the mounting for my Jesse bags uses two slip compression fittings (think bicycle gooseneck) inserted into the rear frame ends... And that’s it! I feel that a mass that large supported so feebly can eventually fail. So, my top case is in pristine condition, sitting on a shelf in my garage. I store things like motorcycle covers in it.