Whatever you do, the dogs WILL alert after some hours. The smell molecules will permeate through the container.Dirty Uncle Ron wrote: 1. Airtight.
2. Handle the item with clean-hands. Residue may transfer onto the item and the dogs will alert, and someone with a good nose as well.
Good luck.
Peace
All these advices are correct, but pack your stuff just before you leave to increase your success rate!
About french trains, I took it a million times. I saw everything, but they are usually waiting for you on your way out of the train, an do some random search. If you are good looking, clean, i'd say you have 80% chances to make it. Last week, there were no cops at all.
Take care.