If it doesn't need to be waterproof, get a simple headphone extension cable. They are available at a certain on-line shopping site and I have seen them in convenience stores on the phone accessory rack.
The one that came with my detector is short too. I just run my hand down it like wringing the water out of a towel. It makes it longer for a period of time so it's not trying to yank the heads phones off your head. I thought about taping a straight stick to a section of it to keep it longer.