One of the keys to getting a big frame in a box the angle. You need to angle the frame to the BB and headtube are along the bottom of the box.....the DT needs to run along the bottom. Doing this lowers the height of the box because the seat lug now angles down sometimes it slighlty shortnes the box because the rear dropouts are moved up and maybe in a little. The pic below is my 'dry fit' and there's one galring problem, the fork steerer is resting against the bottom of the box. I repositioned the fork.
Bike boxes range in width from 7" to over 8", when I pack frames I look for as close to 7" boxes as I can. I dont think chopping a box into a parralellogram helps, you still have lenght plus girth in which case the sender (fedex/usps) will measure H X L X W and lose what I feel are important voids in the box which offer crush protection.
