My apologies for not being clear enough. PhoneNet CONNECTOR is just another type of LocalTalk CONNECTOR. Your clarification is of course enlightening but as benjgvps points out, not very helpful.
In any event, my point is that these boxes have internal hardware and are not just a converter to an RJ-11 connector. The LocalTalk connection module contains a transformer.
As beachycove points out, you do not need an actual LocalTalk connector (PhoneNet or otherwise). You could construct a serial cable by just shoving wires into the mini DIN-8 connectors, matching the serial pinouts. However, that introduces a WIDE margin of error. You need 4 wires, wire pins 3,5,6 & 8 pin-to-pin, instead of the two used by the LocalTalk connectors (and PhoneNet connectors).
As both beachycove and tomlee59 point out, your best bet is to go buy yourself a printer cable or another phonenet box, which can be found quite inexpensively on eBay.
Good luck.