Having trouble making mold with Boolean difference

New poster here. I hope Im in the right place. I am constructing a 3 part mold for a custom container. Basically it is a hollow object with openings on the top and side. The inside is not the same as the outside.

Therefore, I made a center registration part that will allow the inside to keep its shape a stay hollow when filling the mold up. But I cannot get these two objects to difference into a rectangular block.

They work individually, but not together. I was hoping on some insight as to why this is and if there is a way that would actually work.

Dear john4
for boolean-Operation Problems, it is always a nice check to take 2 Objects and intersect them with:
you get some curves that show the intersection of the 2 Volumes. Most times this will also show you the error.
and to check your Polysurfaces use:


an alternative Approach to Boolean:
and if all of this is not pointing you, to your problem:
color the objects / put them to different layers, update your post (e.g. i want to subtract the green from the blue…)
and post your .3dm file

good luck