I've never read about any of the Vietnam POW's building radios. My impression is that they were incredibly closely watched by their guards.
Here is the description from Wikipedia on how Lucas made an earphone. It sounds quite plausible.
Richard built his earphones by binding four nails together with cloth then winding wire and dripping wax over the turns. After about ten layers of wire he placed it in a piece of bamboo. A tin can lid was placed over the coil of wire. The listener connected the improvised earphone to the foxhole radio and received three radio stations.
here is the quote from the article.
But what about the earphone?
Richard Lucas, who was a POW in Vietnam, built a radio in camp and was also able to improvise an earphone. He writes:
Four nails were bound together with cloth from our clothes.
Wire was obtained from wire used around the camp which I might add wasn't coated with varnish. It was bare wire, so we wound a layer and, using a candle, we dripped wax over the turns, which were spaced as closed as possible without shorting out (not touching). We repeated this process over and over again until we had about 10 layers of wire, which were insulated from each other layer by a strip of cloth and wax. Then we put this in a piece of bamboo and adjusted it so it was about a 1/32 of an inch from the end.
A tin can lid was positioned over the coil of wire and nails. Then connecting it to our "foxhole radio" (basic design as yours) we could here about three radio stations. Our antenna was the barbwire around the camp and the ground was wire laid along the ground to make up the ground. Best listening was at night and it had to be pretty quiet because the earphone was pretty weak. If we had a magnet to set up a bias on the coil, the volume would have been a lot louder."
And Mike Barnard points out that "the headphones were almost always acquired from a tank crew's radio operator, and often one side of the headphone was cannibalized for wire to wind the tuning coil while the other was used for listening."
