How about a tarp with plenty of grommet holes and some bungee cords to hold it in place. That would give you a fairly weather tight door with minimal cost but takes a little longer to open and close. If you want to enter it frequently, the only other option would to build a stick wood frame that you can fasten to the face of the shed and then stud it in for a door. It doesn't look stout enough to me to just hinge a door to what's there and have a full width door cantilevered out.