Adding Voronoi to a Brep

Hi! I’m currently looking to create a porous structure as shown here:

Instead of using a marching algorithm, I have chosen to apply a Voronoi pattern (due to lacking experience with Grasshopper I thought this would be easier). However when applied, it assumed a rectangular shape rather than resembling my brep at all:

I followed a youtube tutorial for this:

I have tried making my own Grasshopper script from scratch but this is all I have:

Any suggestions for progression would be much appreciated, thanks!