If you could get ahold of a couple of mattresses to lean against the wall(s) you share with the neighbors, this would do the trick (although it might not look too pretty, it’s only necessary until your kitty gets older and chills out a bit).

Also, it’s most important to cover the level of the wall that the sound waves hit most directly. If your kitty stands oin the floor and looks straight ahead or slightly downward when she meows, you should focus on the lower part of the wall(s). (The upper part won’t be hit as hard by her meowing sounds.) But if she gets on top of a bunkbed or in a loft and then screams, the upper part of the wall will need to be covered as well.

The other thing would be to hang a couple of thick comforters on the wall (you could use halfway nice-looking ones that don’t look weird together). You can get old, cheap comforters at a Goodwill or Salvation Army store, and then you can donate them back when you’re finished with them. Rubber floor covering (like you see at gyms) would be even better though.

Best of luck to you and your kitty!