Your original design measure 7″x7″x7″ internally.
My tweaked design measure 7.125″x7.125″x7.125″ internally.
So, an eighth of an inch bigger internally in each dimension.
I am unclear what you mean by:
it will hit the mechanical limits of the passive radiator sooner.
Can you explain that, please?
Also, do you use SketchUp or something similar for woodworking design?