The P3X fits into the bike box fine, you just have to remove the whole basebar and the monoriser + extensions. You end up securing the basebar to the bike frame on one side or the other. It's pretty easy, just put something firm (like cardboard) in between the bike and the handlebar.

This process would be the same with the S5/P5 Disc.

I also like to use this whenever I'm packing a bike, it's without question the best material for securing things and preventing them from moving. You can find it in many places (Office Depot, Lowes, HD, etc)