The most important thing is to pack your model such that no part touches the container. Even light contact can cause paint to scuff or parts to get bumped off. Cut up sponge foam and block up your model. Something like a car is easy, just a collection of foam sheets that form a block with a rectangular hole, with a separate fitted foam lid to protect the roof of the car. Things like armor with sticky-up bits will need foam block stand-offs between some broad area like the hull top and the lid to keep the model from shifting upwards, should the box turn over. (It could happen so plan for the worst!) Airplanes need to have holes in the foam to prevent things like landing gear, antennas, etc, from touching anything, including the foam padding. Like armor kits, they will need foam stand-offs on the tops of the wings and areas on the fuselage without breakable items.
The container is somewhat less important, it just has to be durable enought to handle knocks and have a lid that won't fall off. I have been using a shoebox with an old belt(!) for several contests and it works well. And if you are flying in, make sure you can carry it aboard rather than checking it in.
Good luck with your build, and enjoy the Nationals experience!