This makes a rounded cubic box of a given width/depth (same value) and height. The roundness of the corners is adjustable according to your preference. There is also provision for a one-line label centered on one face, debossed into the surface. (This has been a bit troublesome to produce, at least on smaller boxes. I'm printing a plain 150x150x180mm box as I write this; perhaps a label would've worked better here.)

Adjust the width, height, and roundness parameters at the top of the OpenSCAD file. For the label, set the label, label_size, and label_depth parameters as appropriate. (label_depth should be kept to something less than your printer's nozzle width.)

Uncomment either body() or lid() at the top, and render to STL. Print in spiral-vase mode with three bottom layers.

STLs for a 50x50x50mm box are included as an example.

This needs a fairly recent build of OpenSCAD to render. I had been using a build for 64-bit Windows from 6 January 2018, but was getting assertions on render until I upgraded to the most recent build (24 January 2019, as of this writing).

Probably a wider nozzle would be more desirable for making strong prints. Failing that, another route to a stronger box would be to forgo vase mode and set 2 or more perimeters and 0% infill in your slicer. On Gentoo Linux, the fkmclane overlay has an ebuild that builds whatever's in the OpenSCAD GitHub.

Update: (28 Feb 19) Corrected the bottom of the box to the shape it should've been, and squared the top to make these more stackable. Also took in the top of the box less so the lid should be a more snug fit.