Make sure to wear Safety Glasses and Gloves when doing this. The cutting wheel will get hot and send metal dust flying. Also, if they break, they explode into pieces.
1) Remove internals of the marker
2) Place the 2 shells together
3) Using the current charging port as a guide, drill a hole on both ends of the port with a 1/4" drill bit through the right shell.
4) Using a Dremel Tool with a "Heavy Duty Cut-Off Wheel #420" make two cuts to connect the two holes and form the new port.
5) Sand down the edges to be smooth
6) Reassemble the marker with the charging handle in the new hole. File as needed to assure a smooth path for the charging handle.