You could cut and paste a program together from Sheetcam and Meshcam.
It is quite simple to write a program to drill some holes. Good practise for getting used to writing gcode properly.
A hemisphere is also not extremely difficult to program. Lots of good learning there. Basically, you can cut the hemisphere by issuing a series of G03 codes to make the cutter travel around a 360 degree circle. All you need the cad for is to create the offset for the hemispheric arc, break it (interpolate it) into short segments so that you can get a list of endpoints to serve as start points for each G03 command.
If you want it to be more easily done, then you'll have to resign yourself to forking over some cash. Sorry there is no real free way to do what you want easily.
I think cut and paste from the two inexpensive programs might get you a fair ways into what it is all about.