What you need is attic insulation stops. They can be bought in foldable cardboard or pre-moulded styrofoam sheets. You staple them around the perimeter of the attic.

Wether you can physically reach that space through the attic is another problem. Most old homes do not have much space between the wall and roof for a human to fit there and work. Even in new homes they are a bitch to install after the home is built. They are installed when the house in being framed in a new build.