I took wet sourdough starter overseas and back about 6 years ago (before I
discovered that one could dry starter). I put 1/4 cup or so in a small jar
and put the jar inside 2 zip-lock plastic bags. It was July ... it
expanded and overflowed the jar lid, but was contained by the plastic bags.
I was never inspected in Customs and didn't get to use my rehearsed
explanation, so I have no advice on that subject.
If you dry sourdough and take it that way, I'd leave it in large chunks so
it looks like broken up soda crackers. My usual method is to whirl it in
the food processor until it's as fine a powder as I can get. I would
definitely NOT advise doing that ; )