How do I add an on off switch to the power cord of a 2 wire AC motor
Do not assume that the fact that both leads from the motor are black means that the motor "has no neutral". One of the leads will definitely be connected to neutral and one will definitely be connected to hot. It may be that the motor is designed to have either lead be hot or it may be that one particular lead is supposed to be the hot. Something in the documentation accompanying the motor or written on the motor will probably tell you which this is.

The electrical box you have chosen is designed to be inside a wall, not loose on a table or on the floor. The box has flaps covering openings for heavy #14 or #12 sheathed cables to enter and be clamped. It is not designed for lamp cord. The flap could cut the insulation on the cord. You would be better off an inline switch designed for flexible cord. You can buy cords with the switch already in.

How do you propose to enclose the motor and protect the connection of the cord to the leads of the motor?  

What is the current draw of this motor? You should select the power cord based on the current draw of the motor. Your proposed wiring is stranded wire "lamp cord" to wire this motor. This varies in current capacity depending on its size and number of strands between 3.5 A to 7 A or maybe 10 A if the cord is larger than it appears in the picture.

The switch is supposed to be in the hot wire. Is the plug you have selected polarized, that is, does it have one blade larger than the other so it will only go into the receptacle one way? If not, get one that is polarized. Your wall receptacles are supposed to be wired so that the smaller slot is hot and the larger slot is neutral. The insulation on one side of the cord will be marked or  ribbed so that you can keep the hot and neutral identified as you make connections. The convention is that ribbed or marked side is neutral. The smooth or unmarked side is hot.

Addition Edit

The inline appliance switch is the better and standard way to put a switch in a flexible cord, but if you would want to use the components you already have on hand, below is how I would used them to wire the connection in the switch box. But this is not an approved use for these devices and the inline switch would be.

Drill two small holes in the sides or ends of the box that are just large enough to insert the wires. File or sand the edges of the holes so they are not sharp. Insert the wires through the holes leaving about 6" or 7" of wire on the inside and then tie the wire into a simple knot large enough so that the knot cannot pull back through the hole. Connect the neutral wires with a wire nut and connect the hot wires to the switch terminals.