Recognition of known structures made from player placed objects?

I’m using blueprints because I’ve not used C++ before. To be honest, this game really should use C++ for obvious performance reasons…
What I want is something similar to the crafting system from MineCraft, but with objects placed on the floor as opposed to a crafting grid.
The game I’m making involves performing rituals, often involving placing things in certain patterns. When the objects are correctly placed, the structure can ‘activate’, causing whatever effect is tied to that structure to occur.

First of all, I’m trying to get all my actors to conform to a grid.
I’ll make it so that there is an activator object for each structure, which when clicked on checks the nearby area for the correctly placed other objects and transforms the lot in to the single structure.
For instance, make a circle of chalk runes, place a goat in the middle, activate the goat to summon a demon.

Fair note though: I’m not going to use voxels. That’s way over my head.