Karabiner use DX footswitch for modifier keys

private.xml

<?xml version="1.0"?>
<root>
  <devicevendordef>
  <vendorname>RDING</vendorname>
  <vendorid>0x0c45</vendorid>
</devicevendordef>
 <deviceproductdef>
 <productname>FOOTSWITCH</productname>
 <productid>0x7403</productid>
  </deviceproductdef>
  <item>
    <name>Foot Pedal swap c to CMD</name>
    <identifier>private.foot_pedal_swap_c_to_CMD</identifier>
    <device_only>DeviceVendor::RDING, DeviceProduct::FOOTSWITCH</device_only>
    <autogen>__KeyToKey__ KeyCode::C, KeyCode::COMMAND_L</autogen>
  </item>
</root>

Thank you! I bought it from here https://www.amazon.com/gp/product/B008YPLFF2/ but the vendorid and productid match. And by following what you did above, I've been able to map it to CONTROL_L.