The design is made to take a round headphone jack that is 8mm in diameter and a max of 23mm in length. The area for the jack is longer than 23mm so you can drop in the jack and then slide it forward and secure with the nut.
I purchased the jack from ebay for roughly $2.50 for five of them.
The cap fastens on via the three existing screws that are under the IIIP sticker.
The pin out for my jack is.
- 1 = Copper colored wire (ground)
- 2 = Blue (left)
- 3 = Red (right)