/goop customstructures import <name> <cuboid radius> [w x y z]
It is tedious to write a YML of the coordinates of every block, so here is a command to do it for you.
When importing a structure, you must be looking at its core, or specify the location of the core with coordinates.
/goop customstructures import Arcane_Altar selection
/goop customstructures import Arcane_Altar 4
Graphic description of the cuboid radius argument.
If you have WorldEdit, you may use the keyword 'selection' instead of an integer number, and your selection will be used instead.