I’m familiar with the problem.
This is highly aircraft dependent but what you may be encountering is the socket is international meaning the two vertical narrow slots have become wider to accommodate pins at angles. In this situation the most stable is a UK plug as it has 3 square pins and is much harder to fall out. Consider your wall charger but with a UK C8 cord.
However I agree @ikari04warrior, Anker has been steadily making higher capacity Powercore with ample power for a long flight which recharge overnight. They also pose less risk as you’re not attached the aircraft for anyone to pull on the cord as they move and pull your device with it.
The most long haul flight friendly Powercore is the 26800 dual 12W A input, it’s the lowest cost, recharges overnight and has 3 ports so you can sit and keep phone/tablet/buds going typically well over 24 hours. 3 ports also typically means you’re charging the nearby passenger phone too (although with the virus I doubt you’ll have nearby anyone). It is an old design so alternative would be the current 20000 PD essential, it recharges overnight and needs a 18W PD port. But be aware if you use both ports it’s only 15W which is less than the 26800 dual input 3 output.
Socket may look like this now (example illustration photo)
And notice how volumeness and fall-out it looks compared to