Hi Phil, I've attached the result of a bunch of tricky trims of one piece against the other, starting with a trim of the voronoi piece against a 2D profile curve as shown above, and then extruding that 2D profile out and then the difficult trims of it as well. For the top surface I had some trim problems but since it was planar I was able to cut the edges where the crossed over each other and then build planar surfaces using Construct > Planar. This should hopefully give you enough to finish it off with a few more trims for slicing off the excess width and then joining in the rest of the surfaces to make it a finished solid. Let me know if you are still stuck with finishing it off.

Anyway, this is the main technique for difficult intersections, basically booleans are a combination of surface/surface intersections and then trimming with the pieces to discard automatically figured out by what volume they are contained inside of, followed by an automatic Join. You can kind of almost think of booleans as a sort of automated "batch mode" of trimming. For complex things where the booleans are getting confused, you can instead work with manual trimming and joining at the surface level instead.

The type of thing you had with a lot of overlapping surfaces tends to make for messy surface/surface intersections.

- Michael