Modifies right turn string launchers into left turn. Requires some cutting and assembly. I thought of making this as my other string launcher modification can also do both rotations.

To get it rotating the left direction, the spool winding and coil/mainspring inside of it is reversed.

Assembly of the spool:

This is done by removing and opening the mainspring case inside the spindle (careful with the eyes it may spring out), then cutting a slot as shown in the picture (mirroring the other slot). Flip and reinsert the mainspring (do not wind it yet) and the shaft, making sure the spring is in the reverse direction. Re close the case, with the spring poking through the newly cut slot. Put a bit of hot glue in the end of the spring to the plastic. 

To wind the spring, rotate the shaft in the correct direction along the slot opening, using the launcher itself as a wrench. Reassemble the whole spool and wind the string the opposite way (clockwise).