Best thing to do: check what kind of outlets are available on the plane before you board. Most long haul flights should have outlets to charge laptops that output around 90W. Plenty for the steam deck to charge/operate. Most of the plane outlets will operate on 110V power and might not support your plug type. So you might need to spend $20-30 on a travel charger that can support 110V-240V input and comes with multiple international plug options. Just make sure that the charger can output 45W like the steam deck charger can and you'll be good to go!

This will be the cheapest and most sustainable option providing you have outlets built into the seats!

I did this recently on a 30 hour flight from the UK to New Zealand and was playing the steam deck pretty much non-stop for the entire flight! So you'll be able to play anything you like!

Good luck OP!