I recently had the privilege of dealing with Chris Taylor of Taylor Made RF who supplied me his rather excellent custom keypad for the Kenwood TS990s.
Chris gives the device the part number TMRF TS990KP and it follows the convention as outlined in the user guide 16-7 of the TS990s user manual which allows for the user to program each of the 8 function keys a specific task, for instance; change antenna, play a recorded voice macro or change filters. An extremely handy accessory.
Of course, you could build one of these but for an “appliance” operator and for sheer mini-bling, the TMRF TS990KP is a delight to use. It comes very nicely packaged with basic instructions, a simple 3.5mm jack to jack stereo lead and of course, the engineered keypad.