This OpenSCAD source contains a few simple functions to make several kinds of rounded rectangular prisms. Rounded corners, edges, or even both.

This was inspired by WilliamAAdams roundedRect function, except that I wanted to try to make rounded rectangular prisms that were the size I specified, not grown by the size of circle's radius.

Feel free to make use of it if you like, or improve/fix them as needed. Caveat emptor, as I've only tested these in a few situations.