I will be moving to another country by plane, and I need to carry an important document (diploma from university) which has a nonstandard format: it is square-shaped and won't fit on A4 folders.
How can I safely carry it?
In any case, if it is not framed, I would consider rolling it up and putting it into a cardboard picture tube. There are some with a fairly large diameter. Most universities send out the diplomas in such tubes anyhow.
Otherwise, get 1 envelope, 2 layers of hard cardboard, and a larger envelope, best a padded one.
You put the diploma into the envelope. Then you tape the envelope on one of the carboards. You then tape the second cardboard on top of the first so that it covers the envelope. Then, put everything into the larger envelope.
If you cannot find the appropriate envelope, you can fold some stiffer paper by yourself and form the envelope with tape. Be careful with glue (I would not use it) since it might leak and damage the document - unless you can prepare everything well in advance so you are sure the glue is dry.
Put your name and your address on the outside.
Then, hand-carry it or put it in the middle of a luggage. Forgetting it in the airplane can be just as much of a risk as the luggage getting lost :)
Source: When you have photos printed in larger sizes at a professional shop, they package the prints like this.