This looks like a great project. Several field exchanges has solutions for that, so the first one I think about is the US SB 22 PT
Here is a link with good pictures. The jack is made to accept ordinary 1/4" stereo plugs, but the wires has extremely small diameter. (2 mm??)
http://www.signalcenter.com/commo/eb...d1/3cord1.html
Another I know is the German one with shorter plugs, but a more normal wire diameter:
https://www.finn.no/bap/forsale/ad.h...kode=155353583
(The seller in this deactivated ad has tried to sell a number of these for some time, and may have more of them stored. )
dsk
PS How many units do you need for this project (no of coils)? DS