Have travelled with large boxes in the past and no problems - though if pushing towards the max dimensions would always end up sent to the oversize area.

Double wall worked fine, but triple would be even better.

On size, cutting down to ensure you are in the dimensions is a good idea (or get the right size in first place),. What you don;t want is someone measuring it and being too large when you are at the airport (willing to bet the people dealing with the large luggage will have a well tuned eye on what is an ok size.