There is a simple solution to this that I have already prototyped - based on others' work.
Materials needed:
Arduino Uno or Mega
USB Host Shield
2 x 220 ohm resistors
MIDI DIN socket
I have purchased Arduino boards and a USB Host Shield from eBay that work. This brings the cost down to a more reasonable level which should be cheaper than a commercial product.
Insert the USB Host Shield into the sockets on the Arduino. Download and install USB Host Shield Library 2.0 and install it in the Arduino IDE. Circuits@Home built the original shield and you can find a library for it there.
Here is the sketch I used with an Arduino Mega:
File > Examples > USB Host Shield Library 2.0 > USBH_MIDI > USB_MIDI_converter_multi
Connect your MIDI DIN socket using the example from this page:
https://www.arduino.cc/en/Tutorial/Midi
Make sure to disconnect the TX pin from the circuit when uploading your code to the Arduino.
If you want to connect more USB MIDI devices, use a powered USB hub and edit the code accordingly. This was actually a very easy Arduino project and should not be very hard for a beginner. There are a minimal number of parts.