has anyone been able to find an aftermarket trunk

Yep - but if he tells you he bought if from Triumph or Jack Lilley etc he's sinning again... he's prone to tellin a story or two :?
 
J.C. Whitney sells a quick disconnect Givi knockoff for $79.99. Mounts easily to the Triumph Rocket accessory rack. I've had mine for 9000 miles now, and it works just fine. No way in hell would I pay Givi prices after using this thing. JCW even shipped to me unbidden and free of charge a metal mounting base to replace the plastic one the trunk came with. I've had no reason to change out the plastic one though.

Corbin makes a universal mount trunk. It's kind of tough to find on their site, but it's there somewhere.

Tsukayu makes a large trunk for universal fit.

Hepco Becker also has trunks available.