I fitted the R380 seatbox to mine but the standard series tunnel won’t fit that. Not sure if an LT77 would work with it 🤔
You could remove the tunnel flange from a series seatbox and graft it onto the defender one though, that could work.
To clear the passenger side chassis the under seat cubby box had to be modified and narrowed but I managed to retain the battery tray.
Or replace your rotten under seat box.