The OP did not mention if a permit was obtained from the Polish consulate to bring the urn into Poland. While working in KTW ten years ago, my seatmates from Calgary had Dad in an urn stowed under the seat in front of them on our LH flight from FRA-KTW. The two daughters had a certificate from the Polish consulate permitting importation of the urn. The urn was closed with a wax seal from the consulate. After entering the country they had a limited time to have the urn interred properly in a cemetery. It was one of the most memorable in-flight conversations I've ever had. Also, contact the airline(s) involved. This might be something new for you but you're certainly not the first passenger to carry an urn. If you go through the proper channels there shouldn't be any trouble. Just keep in mind that several European countries are very sensitive about human remains. Many will not let you keep ashes in the living room or scatter them randomly in some park. Contact the Polish consulate nearest to where you live and call your airline for specifics.