About 890 metres, I think.
Neptune is 4,500,000,000 km from the Sun. The nearest Star is 4.24 light-years away, when a light-year is 9,500,000,000,000 km roughly. This makes that star about 8,900 times further away from the Sun than Neptune is, so that 8,900*10cm = 890m.
So you'll need a pretty big box.
The distance to the edge of the Solar System depends just a little on what source you consult, but could be anything between 120 times the distance between earth and Sun) and 100,000 times that distance.