Little boxes, with an optional fitted-lid.
I explored this for Chinese flash cards.
I take a US letter (8 1/2" x 11") sheet, and cut it to 8" x 8", and use that.
I used the instructions from origami-make.com.
The instructions could be done better, and I could easily create a better tutorial, but I'm not interested right now.
To make a lid, I took the exact same box instructions and adjusted just one step.
When folding the square to turn it into a rectangle, don't bring the edges completely to the middle. Imagine that when you're folding, you're making the wall of the box. You're basically making that wall a little bit shorter, giving more room for the base which the walls surround. You're essentially making a box that's a bit wider but less tall.