Standard MIDI converter for USB-only MIDI keyboard

MIDI devices like inexpensive master keyboards are no longer fitted with the standard 6-pin DIN MIDI OUT socket. They have only a USB port. My applications involve the Arduino MIDI library, which relies on serial communication, usually via an opto-coupled interface to the MIDI-standard current loop. This old technology is by no means obsolete. Using a PC with a commercial USB-MIDI interface would be a clumsy workaround, subject to latency. I think a converter from USB MIDI to current-loop MIDI or serial would be of interest to many people, but unfortunately I wouldn't be able to make one myself within a reasonably short time.