Can I give you an advice that might be helpful? Don't plan, measure and cut for an untested project when having great headaches!
And maybe you shouldn't make your notes and calculations on a small piece of paper like I did last Wednesday:
Because if you do so, you might end up sewing two projects instead of one!
I planned to make another divided basket, this time with two dividers and using the technique I detected for the picnic bag. After planning the final dimensions, making a little (or better: tiny) drawing, calculating the sizes of the pieces to cut and cutting, I happily began the sewing and ended up with a nice inside basket, divided into 3 sections like planned (only two compartments have been 1cm longer than I wanted, because my head wasn't capable to divide 13 by 2) and a nice outside basket. And only when I wanted to combine the two, I recognized that one was about 13cm longer than the other! I forgot to add the width of the basket to the calculation of the outside pieces! Stupid headache - next time I do mending socks, when my brain is taking a timeout.
Anyway, today I cut again and produced a simple divided interior for the small outer basket and a large outside for the double divided inside basket.
The long one is for our bathroom to tidy up all the things that are always lying around on the wall tray.
I don't regret having the other one too. It is a spare present and I think I knew already who will get it.