This is my first OpenSCAD part. I decided to start with something fairly easy, in this case a box and lid. If you have any suggestions, feel free to leave a comment.

To create a round box use the same value for width and length, then set the corner radius to half of that.

The script will always generate both box and lid. I might change this in the future. For now, you can just split the 2 parts up in your slicer, then delete one of the two and print them separately.

Updates:
21 February 2016 - Added option to customize section height.

Print instructions

Print Settings

No supports needed, for fault settings with thin walls, use 100% infill.

Notes:

The defaults are optimized to be used with a 0.4mm nozzle, but those can be easily modified in OpenSCAD.

For a snug fit between the box and the lid, use 0.4 as tolerance. Depends on your printer accuracy. For me 0.4 was a perfect fit, but for you 0.4 might be too loose.