Remember you might use the box for in country travel too. Check the dimensions of the box you intend to buy against UPS / Fedex or what ever you use in your country. I picked up a used Team case and it is about 1 inch over the oversize limit for shipping companies.
Airlines also have size limits. It pays to do your homework. We FEDXed our bike on our last tour. My bike cost $100 more to ship than my wife's bike because I exceeded the seemingly arbitrary size limit. Her 47 cm bike fit in a much smaller box than my 58 cm.
I have had pretty positive experiences with TSA. I don't put much tape on the side I want them to open it from, and if possible talk to them while they are looking in the box. Machka's advice about smiling and being friendly is good advice.
All of the above suggestions plus a few.
Cardboard works fine for one-way travel. This box was on 5 different flights to get us to the start of our tour. While it looks beat up , the bike was just fine. The empty pannniers and helmet go inside the box.
Don't forget the tools to put it back together at your destination.
