This is a remix - slight modification - of norm8332's SIO plug. I scaled it to 95% to print in PLA and changed the fastener to an M3 x 10-mm cap head screw with no nut.
The first one I made failed because I wasn't careful with soldering the wires to the contacts - I made the mistake of too much wire length protruding under the spring. The correct way is shown in the photographs - no wire under the spring. Seems to work.
I used 7x20 gauge sprinkler system cable. It is a little stiff, but I was too impatient to order and found this at the local home improvement store.
Print in PLA. Use a brim to stop warping. If you want to use ABS, use the original design from norm8332.