Void sweep in Dynamo

We are a Group of students, who are trying to parameterize a Concrete slab in Dynamo. We need to make a void sweep extursion around/underneath the slab as shown in the Picture.

I have tried to use the node “Surface.BySweep”, to make the geometry of the extursion. Are there any nodes which could make these Surfaces into a solid, and then into a void extursion?

Thanks for any help!

Dynamo script:
void sweep.dyn (49.6 KB)

Maybe this will be useful? https://dictionary.dynamobim.com/#/Geometry/Solid/Create/BySweep

Also a Polycurve.ByJoinedCurves node may be a better use than the curve.join node.