Fusion 360 is free, and handy if you want to make engineering objects. Proved to be really difficult for other things.

- First, we need the radius of a simplex thin lens of f=4cm. The refraction index of the clear resin is close to 1.5. There is a handy calculator in Hyperphysics for the Lens-Maker’s formula here.

The formula reads:(1)

I set and for a cm, the cm. - In Fusion 360 sketch, I draw a hemisphere of cm.
- I place equally spaced lines from the bottom until it connects with the hemisphere
- I sketch over arcs tracing the hemisphere between the segments.
- I remove the hemisphere and the lines
- For each segment of the arc, I remove the coincidence with the next
- I move the segment down to the lower plane with point-to-point move tool, and create perpendicular lines

- The final sketch looks like this:

- I revolve the sketch to obtain the 3D printable part