I've got an old curvy pillion seat which was modded to fit a topbox base. Had a 50L box which I used for commuting and trips. Just stick the normal seat back on for weekend fun.
Only picture I can find of it fitted to the bike (sans topbox unfortunately)
These days I use a US20 for most day use and an Oxford Aqua for trips away.
The US20 is nearly 10 years old and has never let any water in, is easy to clip on and remove (so security may be an issue depending on where you leave the bike).
Edit: found a post about pillion seat adapter:
https://r.tapatalk.com/shareLink/top...ink_source=app
#21 gives details of the mod