Rafał Jankowski has announced he will be releasing a new multicart for the Atari Lynx called the AgaCart. The AgaCart will use an SD cart for storing ROM files. You choose the games using buttons on the back of the cartridge. The Cart supports 256k and 512k games in .lyx format, 128k files need to be converted to 256k. The maximum number of files is 199.
Rafał Jankowski has released already a 48-in-1 Lynx multicart in the past, so he has experience with this type of project and this is essentially an updated version of his previous multicart.
For those interested, the AgaCart can already be ordered by contacting Rafał at the Polish Atari Forums. He plans to start a thread for taking orders on Atari Age as well. If you're curious about the technical details, check out the thread at the Polish Atari Forum.