Look for a shirt pattern with loose sleeves gathered at the wrist like this
, and with a rounded neck. Then you'd only need to add the shawl collar.
Be careful to make the neck opening wide enough, since there are no buttons! If you adapt a pattern that has a buttoned front, check that it's still loose enough to be put on and off comfortably. If needed, you can add an invisible zipper on one side to add room (it doesn't have to open completely, but go from 1-2 cm under the armpit to 1-2 cm before the bottom of the side seam).