Not long ago, I needed a pen size box and was really impressed with Almisuifre's Minimalist Pen Case with Tab Closure. Recently, I needed another box and wished to have one in a similar style so I created a model along similar lines but that could be configured to any size. Model is included so it will work in the Makerworld customizer. Two changes I made are to make the side indentations a little deeper for a more textured effect and I mad the click close bump a little more robust. I think I may modify that at some point as it's a little tricky. If you have problems with it, a little light sanding may help.

Minimum size for the width is about 25mm though it might go smaller, it probably won't be that useful at that size.