This FM transmitter kit can be installed in a jar or any other enclosure limited by your imagination. It's $25 in the Makers Market / Boing Boing Bazaar.
This bare bones kit provides everything you need to build Mr Kogawa's Micro FM Transmitter, excluding the enclosure, giving you the freedom to build it into whatever case you choose.Mr Kogawa's Micro FM Transmitter – This is a short range FM radio transmission kit, it can be tuned to broadcast on any part of the FM range and is amazingly clean and stable. A connection for an optional external antenna means that you can boost the range if you wish to do so. The original design of this transmitter circuit was created by Mr Tetsuo Kogawa, he is an artist, teacher and all round guru of Micro FM.