I'm sure to have one of them, but I'm not sure where it is ;-) so I decided to DIM (do it myself!) an N3 plug.
Step 1: The reference
Step 2: The pins guides
Bend the wires so that they will remain in the mould after the casting, I've also glued them together with some double component epoxy glue.
To avoid that the plug sticks to the cast you can cover it with some silicone grease.
Step 3: The silicone rubber
Step 4: The molding stamp
Step 5: The cables
Step 6: The casting
You can now flow the molding resin, or rubber. I've used black rubber because I wanted that the plug be some flexible, but liquid plastic is good enough.
Step 7: The rough product
Anyway... you can cut away some rubber in excess, and refine the plug so to wrap it into a piece of heat-shrinking tube.