you might check this plugin out : Components colission detection or checking if point is inside solid - #3 by eneroth3
But :
- it would be for solids, not rectangles. You can easily, even if temporary, turn your rectangles into solids by making them components.
- it’s not going to spot where you can fit more rectangles, only spot if two or more are in conflict.
I guess you’d need another code combining mathematics to do that - ruby would be appropriate and sketchup a good media to illustrate it, but perhaps it would be easier with another language / software dealing with 2D only