When travelling, I just use a small plastic water bottle and decant the needles into my sharps container when I get home,
I inject whilst in my seat in a plane. It is much more stable than injecting whilst on a train.
As
@EllieM says, keep all diabetes kit in your hand luggage and request a letter for travel from your diabetes consultant (a GP may charge). I have needed my letter a couple of times in over a hundred flights but would rather have it and not need it than not have it when requested.
I only mention diabetes when asked. This includes telling the luggage check in when they ask if I have any sharp items in my hand luggage. I have never needed to tell security or the plane crew.