Wiring a JUMP Interface for multiple controllers is not much harder than constructing a single-port interface. Since the input lines are the same for every controller (power, ground, clock, and latch), you can either connect each of those input wires directly to the circuit board or you can daisy-chain the JUMP connectors in a series. I prefer the latter technique because it causes a lot less clutter inside the case. Basically, you start by building a single port interface. The only major difference is for each of the four input lines, you connect another wire which will connect to the next connector. Then you connect the data pin from each JUMP connector to its corresponding pin on the 25-pin port.

Back View of a standard 2-port version

View of a custom interface to be flush-mounted

The data pin connections are slightly different for the Linux version than for the DOS / Windows version. To build a Linux compatible interface, build the interface exactly the same BUT connect the data pins according to the Linux mapping instead ( found on the JUMP Interface Basics page ). 

Click on this image to see view the diagram in actual size
(Thanks go to Paul Drexhage for drawing this diagram)

As you can see, the easiest way to build a multi-port version is to daisy-chain the input lines from one connector to the next. then the only pin which will need to be directly connected from the 9-pin connectors to the 25-pin connectors is the data line (pin 4 of the 9-pin interface which connects to different pins on the 25 pin port. The diodes are standard type 1N914. And remember that you can include any number of gamepad connectors between one and five. Just  chain on another connector and connect the data pin from the 9-pin port to the correct pin on the 25-pin side.

This is a blurry picture of what the inside 
of the interface can look like.

As with the other interface designs, the wiring is simple. The most difficult part for me was trying to get everything organized within a case which didn't look too sloppy. 

You can pick up a project box from RadioShack and you should be able to find one that will house your interface the way you want it.

For the PC-side connector on my first interface, I just cut a printer cable, stripped all 25 wires inside and hunted down every wire which corresponded to a pin I needed to use...I don't recommend following my example there. Again, crimping connectors and a 25-pin interface connected to the project case are the simple way to go because then you make all the connections you need and don't have to fiddle with 25 tiny wires. 

To get a better understanding of all this stuff, be sure to read the JUMP Interface Basics page.