Others may know a good source for terminal posts.
Radio Shack sells them. On the other hand, if you're handy, you can make them easily. What you need (per speaker) is the following:
2 ea. #8-32 round headed machine screws
2 ea. #8-32 hex nuts
2 ea. #8-32 knurled nuts
2 ea. 1/8" flat washers
4 ea. 1/8" fiber washers
2 ea. crimp connectors w/ a 1/8" hole
You can get probably all of this at a decent hardware store (not a "home improvement center" like Home Depot). Oddly, you might find it easier to get the knurled nuts in brass so, if you can get the machine screws, flat washers and hex nuts in brass too, that would be cool.
1. So, to begin, remove the woofers and fiberglass. At this point you'll be able to access the crossover plate.
2. Drill 2 holes in the crossover plate to accept the machine screws, ideally, equally spaced on either side of the RCA jack.
3. You can either cut the wires connected to the RCAs
at the RCA or run new wires from the crossover for the binding posts you're about to install if you want to retain the option of using the RCA connections.
4. Put the crimp connectors on the ends of the wires, either the snipped existing ones or if you're retaining the RCAs, on the ends of your new wires.
5. Insert the machine screws through the hole in the crimp connector.
6. Next, slide one of the fiber washers onto the machine screw and insert the machine screw into the hole you drilled in the crossover plate.
7. On the outside of the crossover plate install first another fiber washer, the flat washer and then the hex nut. Tighten.
8. The knurled nut goes on last.
One last thing. Since the Model Twenty is an acoustic suspension speaker, a well sealed cabinet is important. An RCA jack without something inserted into it constitutes a fairly significant air leak. To correct this, just stick an RCA plug, either scavenged from something or purchased new, into the jack.
John