...also, all the 'ends' of the open surface(s) will need to be co-planar...
...(which, if it's a box, they will be, but then, why would you do it that way... :^ )
...(ie, the 'solidising' function ['close feature'] won't smooth a 'free-form patch' thro non-coplanar ends of [joined] surfaces, - that's my experience so far!)
steve