This is how I would do it, the steps will make sense with the video below:
1) Use the pen tool imitating the shape from the outside but with an extra in the right corner
2) Apply a stroke aligned to the center (I've set it at 58 px)
3) Outlike the stroke and remove the extra at the right matching your base shape
Like this: https://recordit.co/4HrNI1n28J