For what sounds quite straight-forward, just some hand coding with a parametric style to adjust dimensions might be easy enough. Mach-x can do this.
Otherwise have a look at:
1. SimplyCam from MR-Soft Nc Software Tools - not free nor expensive but the demo version can produce about 100 lines of code and is otherwise fully functional (except saving drawings)
2. Some of the Linux based CNC software - may be overly complicated for what you want.
Cheers,