European plane adapters

All planes are different, but are you certain that:

A: Your wall-wart (charger) was NOT rated "100-240 Volts AC?"
or
B: The plane's plug actually included USA parallel blade slots in line with the 240 Volt receptacle? You'd need to bend down and use your phone flashlight to get the confidence to push your plug in against the resistance.

Was there a USB option, and do you have that cord handy?

Just trying to offer multiple solutions. Your compartment note suggests this was Business Class. It's hard to imagine Business Class seating that could not accept a two-prong US plug. (I'm mainly United.)

Are you sure that you need an adapter? Modern passenger jets usually have universal power outlets.

Unless you are talking about the charging brick itself - Anker has some that are fairly compact

I'm not sure if it would work in your circumstances but have you ever considered an adapter / extension strip? They usually have much more compact plugs whilst the extension cord allows you to put the more bulky outlet part somewhere more convenient.

Something like this - https://www.amazon.com/European-Adapter-Universal-Extension-International/dp/B0FBR3MB1P

Probably not perfect if you only want it for use on a plane but they give great flexibility and multiple outlets / USB ports for general travel.

I nowadays always buy chargers that have an extension cord. What is usually called a "desktop charger". Unlike wall warts they do not hang precariously from the socket, and work even if the socket is in an awkward spot (like on some trains).

I carry an external battery fully charged. That way I've got power even if my USB or adapters don't work.

Like jkh, I travel with a small power bank, which makes keeping my phone charged during the day much easier.

Thanks for your responses. I did finally find a picture of the plug set up on the plane I am flying, and they have actually put the plugs
on the outside of the cubbies! Smart move. I do have a charging block if I need it next time.