The problem of measuring force of a punch is more complicated than it seems.
The first challenge is to figure out what variable you are trying to measure. You could measure velocity, acceleration, pressure, impulse, etc.
Pressure is force over area. If I concentrate the force of my body weight into a small area like my top two knuckles and punch something, there will be a large pressure at that point. If the force is spread over 4 knuckles and the front of my fingers, the pressure will be lower since the area is larger. Imagine hitting a single rib with one knuckle or hitting a hard skull with an entire fist. The pressure on both the hand and the target will be very different.
But with respect to boxing, pressure doesn't tell you the entire story. For boxing it is more useful to measure impulse or change of momentum. That will you give a better read of knock out power.
Check out "Fight Like a Physicist: The Incredible Science Behind Martial Arts"
Book by Jason Thalken. This book goes pretty deep into the subject of knockouts.
The machines in arcades usually have a punching bag on hinge. When you punch the bag, the bag accelerates until slams into a hard stop.
There a few ways you could approach this problem. You could measure the change in velocity of the bag and calculate change in momentum. You could measure the acceleration of the bag directly using accelerators. You could use a compressible material and measure the deflection. You could also measure the force exerted on the hard stop by the bag. You could measure force over area to determine pressure. It largely depends on which variable you decide to investigate.