This is not a well-known phrase. I'm guessing it's a combination of two things:
"Plato's cave," referring to the philosophical Allegory of the Cave, which proposes that all things in the world are shadows cast by "perfect forms" that exist in some abstract sense.
The attic of the house as being a place where you store all sorts of things that might be useful in the future.
So "Plato's attic of perfect forms" is a metaphorical storeroom where all the perfect forms are kept until a time when they might be needed.
By saying that "he could not imagine" a prototype better than the "copy he loved," the author is saying that Phibba (note that this is a proper name and should be capitalized) is such a amazing woman that the Platonic ideal of the essence of all womankind could not be an improvement over Phibba herself.