How to carry a backpack for a flight 

Very simple, buy the IKEA Frakta bag and put your backpack in it.
My backpack in 70 liters in total and it fits pretty well.
Check the measurements tho before buying to make sure it will fit.
In my case I still have some space left on the sides to fill up the IKEA bag.
Check the links below where to buy all you need:
x1 IKEA bag: www.amazon.com...
x2 pack IKEA bags for you and your partner (btw nice to have an extra one in case you are buying stuff on your trips): www.amazon.com...
Zip ties: www.amazon.com...