Recommendations for rear luggage

Hi all. Just got my first Rocket 3 GT, a 2020 bike. It has the Triumph rear rack fitted. I am going touring in June. Can anyone recommend a spacious rear bag that either fits the standard rear seat or the rack? I am in the UK. Cheers.

I'm not sure if you have them over there, but I always loved the old school "Gearsack".
I still have a few that came with bikes that I bought.
They were a generic soft bag with a simple mounting system. They had an inverted pocket that just slipped over a hoop rail frame. Extremely well made and so versatile.
I used to sit mine on the pillion seat, so imagine that they would work well on your GT. Just slipped over the pillion back rest.