Problem with new bike panniers

I am kayaking tomorrow in my new (to-me) kayak which is stored at my friend Beate's as she has the car.  As this is not a club session, I need to take extra stuff including a changing robe rather than the small microfibre towel as I will be changing in a carpark not a changing room.  I therefore wanted bigger panniers.  I found some that seemed slighty roomier in Decathlon last week, with an extra "pannier" that clips and upclips to the top.  Triedt o put it on tonight to find that the way of fixing it to the pannier carrier is impossible as the ribs attached the pannier carrier to the bike get in the way. Grrr! I don't know what type of pannier carrier would work with this fitting!