I noticed our workers cement mixers are started/stopped simply by plugging them in/out - but the plug is located about 20M away from the machine, so this doesn't seem very safe in the case of something going wrong

The motor is a 3HP single phase Mitsubishi with a capacitor start + run, drawing 15amps under load and 96 amps startup current - what parts do I need to buy to implement a stop/start switch at the machine itself, and how would it need to be wired?

I'm assuming a 16A light switch isn't the way to do this?

Your local electrical shop should have 20A switches in nice simple boxes that you can use. The problem will be longevity under the control of the locals, the switch will be physically broken in no time.

More expensive would be something like JesseFrank posted, in a box of course, or a push button starter. Both are mechanically more robust then a simple switch, but, you'll probably be needing a waterproof unit in case it rains = more $$$

Probably best to browse around the specialist shops (assuming you have one of course).

Connection is easy, simply break the flex close to a suitable mounting point on the mixer and insert the switch.

To be honest, if the workers are happy pulling the plug they won't thank you for any trouble or expense you go to

Ended up buying them a few of those cheap 'blade' main breakers used in Isaan houses - does the trick