I have a quick tip to add to this.... When connecting these up with kettle plugs, they very often get hot. Sometimes, this is due to arcing (electricity jumping through the air) with gives a lot of heat, even to the point of the plug melting. Very often this is caused by bad connections. I found a very cheap way of making a rock solid connection without hacking the element.

Use a 30a connection block. If you get the right size, the screws and opposite side of the prongs make excellent contact. Is rock steady and no arcing. Tested successfully for prolonged use. Usual disclaimer applies - take responsibility for your own safety!!! Check what you are doing is safe, don't rely on this alone! Earth connection as other designs, or to connecting metal on pot.
I've also built a neat shroud to wrap round all of this, may post with installed.
Hope this helps.