Does anyone have any recommendation for creating a barrier on a wall that would soundproof or at least muffle sound from a neighboring apartment, short of retrofitting sheetrock using green glue or quiet rock?

I have heard that there are panels that can be retrofitted fairly closely to the size and then you can take them off when you move.

Since you will need air tight assembly, I am not aware of a portable soundproof barrier. It is hard enough trying to soundproof a wall, ceiling, and floor using permanant type of construction to an already existing building. Question, are you a owner or renter?

I have never used this product but you might want to check out this and click on the "DesignWall Interior Panels". Goodluck in your seerch.

http://www.homasote.com/products.html

I've heard lead sheets work well but the edges need to be sealed tight. If you are renting, I feel for you. Been there and if the neighbors are unreasonable, the next best thing would be to move.