At home I was using a four way adapter and rarely removed my charger, then I went on holiday, using the wall mounted socket in the hotel was evil, this needs to change before I go away again!
Step 1: Prepare the Plastic
To ensure the Sugru makes a good bond with the plastic use a file of some sort to roughen off the plastic between the existing finger holes and the top flat surface.
Step 2: Adding Sugru Handles
I used about 1/3 of a packet of Sugru for each handle, you can probably get away with 1/4 of a packet. Roll each bit into a rough ball and press onto the side of the plug, use your finger to spread the Sugru partially onto the front surface and down into the existing thumb holes. Also spread slightly out to the side.