I just used normal styrofoam, from inside a box of somethign I bought. I dont even remember.

I put the window almost all the way up and glued styrfoam on both sides of the small part of the window that was still inside the door. Well, not ON the window, but on the door, but you get the point. Tight fit but now the window has no where to go as it rolls up and down. Im going to redo it with something better, sometime. No reason to until it starts rattling again