I've got a 2021 Mini ST2 with the small Rad basket on the back. Love it, sturdy basket, but I've been carrying a battery-powered tire inflater, bungee cords, rain bags for seat/display etc in a waterproof bag and it was taking up almost half the volume of the basket, leaving little for grocery bags etc.
Finally got smart and a bit creative, so I ordered this handlebar bag ( https://www.amazon.com/dp/B0BRXFWCDB ) which straps to the top side rail of the basket and holds it securely in place. If I need the basket space, now I can just flip it over without disturbing the straps at all, so it hangs outside temporarily like a pannier. Simple and effective. This is the "cuboid" version, I think the "cylinder" version is a bit too large (even though rated for a bit LESS volume) but I didn't try it out. It seems well-made with strong straps and good stitching and appears very water-resistant.
This suits my needs perfectly; see pics below. Might help you, too.
Finally got smart and a bit creative, so I ordered this handlebar bag ( https://www.amazon.com/dp/B0BRXFWCDB ) which straps to the top side rail of the basket and holds it securely in place. If I need the basket space, now I can just flip it over without disturbing the straps at all, so it hangs outside temporarily like a pannier. Simple and effective. This is the "cuboid" version, I think the "cylinder" version is a bit too large (even though rated for a bit LESS volume) but I didn't try it out. It seems well-made with strong straps and good stitching and appears very water-resistant.
This suits my needs perfectly; see pics below. Might help you, too.