I copied and pasted this, hope it works
It's off an off-road BMW650 I understand. I have a hatred of top boxes unless Mrs is with me. I usually ride solo and live on the bike during the week, so I try to optimise capacity and avoid putting it all at the back and up high.
Ebay find for £20. They are usually for Honda, Yamaha or BMW emergency services bikes.
Fitted mine on a flat alloy plate I cut to follow the subframe.
Front locates in the helmet hooks and the back has a staple to mimic the rear seat lock. Whole unit lifts off with the key like the rear seat.
There's a 150mm square hole in the plate so I can get through to the under seat storage.
Mine isn't an SE so I utilised the connector for the rear Hella socket so I have power inside the unit for phone etc.
It holds roughly the same as a top box but you can't get a bike helmet in it. It will hold a hard hat, boots and tools easily.
Hope that helps.