I have always found that eggs sent in polyboxes have a lower sucess rate, for whatever reason and consequently prefer to buy from sellers who don't use them. I suspect they somehow "suffocate" the egg, so if I were to do it myself, I think I'd find ones with extra large holes and wrap them with something breathable to stop rattling around rather tha a tight fit. Alternatively it could be they get more jarred, in which case extra padding would also help.
But doing it that way, there isn't much point in the polyboxes in the first place, so I stick to more traditional methods!