I attached a model showing two ways of doing that. Both use a mask and mask parameters to allow change of parameters afterwards. The difference between the two ways are how to represent the hemisphere. The simpler way is to use two spheres, with two halves overlapping with the cylinder. A fancy way is to create hemisphere using revolved solid. The mask provides a way to create the curve for creating the hemisphere (see its initialization tab).
You may replace the "fixed" cylinder with variable cylinder. The idea would be the same.
