Measure the wall to determine the width you want your coat rack. Obtain a solid wood board -- either a new piece from a lumber yard or an old piece from a salvage yard. If the board is too long for your wall, cut it with a jigsaw.
Lay the board on a table. Sand the board smooth with a fine-grit sandpaper. Paint or stain the board to a color that complements your decor, and let it dry.
Place some dummy spindles on the board, and space them out. Lay the door plates or rosettes over the dummy spindles so the spindles protrude through the holes. Slip the door knobs over the spindles.
Arrange the door knobs on the board until you are satisfied with their placement. Make pencil marks to note where you want the plates to go.
Remove the knobs and the plates, and mark exactly where the dummy spindles will go.
Put screws into the holes in the base of the spindles and screw them into the board. Put the plate or rosette over the spindle and screw it into the board.
Apply a silicone adhesive to the spindles. Slip the door knobs on them to secure them. Once the adhesive sets, your door-knob coat rack is ready for use.