This one fits perfect under the seat of my 2013 GSA........... ABUS Granit X Plus 540/160HB300 U-Lock. I spent a lot of time researching measuring etc to get the best I could find that fitted.
The U-Piece slides under the pillion seat in the plastic channels, and the Lock fits in the tool tray (with a microfibre cloth underneath). I got mine from Santafixie back in March, link below, was just about the cheapest place I could find it
When I bought mine there were two choices, one with a bicycle frame mount for a couple of quid more the other just the lock, looks like the only option there is now is with the bicycle frame mount...