Voronoi edges on Box (/Brep) faces

Hi all,

Apologies in advance for the n00b question; I'm just trying to get back into GH again after a long downtime.

Having searched around for a way of selecting the parts (vertices, faces, edges) of a volumetric voronoi diagram (Voronoi 3D) where they intersect with a Brep, I finally stumbled across this tutorial: https://www.youtube.com/watch?v=ZXt6WgqzeIM

However, when I try to emulate it, I do not get the nice result that the author of that video obtains (on the left in the attached screen grab), but the less-than-satisfying geometry on the right.

I understand I'm doing something silly here, but I've been staring at this for a long time now; if anyone has a spare five minutes to let me know where I'm going wrong, I'd be very, very grateful.

Apologies, again, for the probably not very accurate notes to self - just trying to keep track of what I'm doing throughout each step of the process.

Many thanks!

m