Corners for a child.

 I saw how he admired the drape of that sheet, the lines, the tracks it created, the ideal marble roll. I heard the deep satisfaction with how it rippled in his voice.

 Draping that sheet over the chair and stool and table made a room.
 It was all nailed down with a jug of fabric softener and rocks.
 He built a room of small corners. The corners are so small "that only a child can fit inside".