I use this -
https://waldsports.com/store/front-b...33-q-r-basket/ from Wald.
Not pretty, but it carries a fair amount of stuff. And I can ride the bike to the grocery story, take the basket off, and use it for shopping. Then just put it back on the bike when I'm done, and ride home.
I use it when I ride to work. I put my backpack, with my laptop and other stuff, in the basked. Lock up the bike, then carry the basket up to my office.
Works great. And it was pretty inexpensive.