Closing trimmed voronoi curves?

I've (along with a colleague) created a fairly rudimentary city building voronoi command, that populates a given region with a random scattering of points, and from those points creates a voronoi which is then in turn trimmed using the region inputted at the start.

At this stage everything works smoothly (picture 1). Using these curves I then want to offset to create a road network, offset once more to create a building depth and then loft to create the final building.

Unfortunately the voronoi curves along the edges are open curves from where they were trimmed, and as a result the offset fails to work correctly (not suprisingly) but I can't seem to find a method to close up the curves using the bounding area to close them? I've tried region difference/union/intersection as well as join curves to no avail.

At this point the only thing I can think of is to avoid the trim, check where the curves intersect and create points and then use those points to create a line and close the outer curves in that way, although that would ruin the curvature of the region.

Any suggestions?