Just throwing up a few images of my extrusion head...
The bit coming out the end is remnants of HDPE from milk bottles, the iron rod coming out the other side is what I used to try extruding and to force air out from new plastic, and the leads go to the nichrome. The cork is to hold it onto the power connector of the 14V battery pack I've been running it on.
How I made it is by first forging the tip of the pipe down as much as I could (I'm not very skilled with copper, and ended up cracking it a lot in doing so - didn't heat it enough.), then painting on a thick batch of plaster of paris to the pipe, letting it set, wrapping the nichrome around it in stages, putting more plaster on the newly-wrapped part, letting it set, etc. Afterwards, I ran the tip end back up over the plaster, put one more layer on, and let it set.
The two curved bits of plastic below the extruder are what happened when I tried it. The rather ugly bit on the left end of the lower one is the first bit I tried to extrude - didn't work very well. The discoloration is most likely from the method of reshaping the copper, which left it rather dirty inside. The rest of it was me drawing the plastic out of the thing by holding it down and pulling on the plastic with pliers.
...the things up in the corner are just some of the tools I've been using for various other bits of my effort, made with the forge out back, two knives and a compass :)