Boolean Blunders: A heartfelt request.

Hello everyone! I am struggling with a model for print that… I just cannot seem to wrap my head around for whatever reason and I was hoping to glean knowledge from those more experienced than myself.
I am working on a mold of sorts involving a heart. (heartfelt request? Eh? Eh… okay.) It is going to require several layers and variants, and I just do not seem to be able to figure out a way to make one of them work. I have access to and decent experience with Zbrush, Blender, and Tinkercad, so any workarounds that are compatible with those would be super helpful.

In the example above, you can see that I have the organ booleaned out of a solid block. But the trouble is, now I need a hallow version of /both filled areas/ with the empty spaced filled. Essentially reversing the polarities as it were. I am certain there is some sort of obvious process for this but for the life of me I keep just making a mess of the thing. Would anyone be able to shine some light on this dillema for me and potentially help me solve it? I would be deeply grateful.

Thank you,

I’m not able to see an image in your post. I think that’s specific to new users with under a certain number of posts, but I can’t remember.

By any chance are you talking about something like the boolean modifier in blender? I think you can make a hollow in an object by using the difference option in the modifier’s drop-down menu.