maybe you also want to look into stuff like this: https://www.amazon.com/Delta-Mega-Rack-Bicycle-Cargo/dp/B000ACAMII
I've heard some people use one of these to attach to the top. That assumes you have appropriate points to attach it to. https://www.amazon.com/dp/B000ACAMII
I got this one today. Fits good but maybe I should’ve gotten a 15x15. We’ll see.
Actually I forgot to mention I use a cargo net too, in addition to the tote bag. I have this one: Delta Cycle Bike https://smile.amazon.com/dp/B000ACAMII
I spent an hour or so putting this little wooden crate together. I think it turned out pretty well for a small cost and gives me lots of flexibility being completely removable.
The crate is this, they have tons available but I went for one that fits as well as I could find. Only a small chunk of the box needs to be sawn out to get a really snug fit.
I then painted it with a few coats of black spray paint and then finished with 3 clear matte spray paint which claims to have some waterproofing capabilities. It's been on my bike for about a month through December in the UK and so far so good.
I like it as I can remove it in about 20 seconds by just removing the elastic strap and popping the crate off if I need to take wider/taller/generally larger items. I use this little crate to run to the shops for milk/bread/beer and it's proving really useful.
I then grabbed a small net off Amazon to stop things jumping out over the bumps. It's a little tight but is pretty perfect all things considered.
Hope this is useful for a few of you. For around £20 you can get a nice little crate to help you with your shopping.