speakerphone for 2.5mm headset jack

I've searched high and low and can NOT find a speakerphone to plug into my headset jack on my cordless phone. Does such a device exist anywhere? I really didn't want to spend more than 20 or 30 dollars on it, as I could just buy a new speakerphone enabled cordless for that. Conceptually, it seems that such a contraption would be easy to make and I'm very suprised that it's so difficult to find as headsets for cordless phones are EVERYWHERE. If anybody has any ideas or suggestions as to how to find/make one, I'd really appreciate it. Thank you.