Ideas

First off, a new adapter for the WiFiChron or HDSP clock. The 8-LED Neopixel stick from adafruit has the perfect dimensions to fit them. Naturally, the time is displayed in color code. The best definitions for the 10 colors I could come up with are these:

byte color[10][3] =
{
  {0,0,0}      /*black*/,
  {30,9,0}     /*brown*/,
  {100,0,0}    /*red*/,
  {128,60,0}   /*orange*/,
  {110,120,0}  /*yellow*/,
  {0,64,0}     /*green*/,
  {0,0,111}    /*blue*/,
  {64,0,63}    /*violet*/,
  {15,15,15}   /*grey*/,
  {127,127,127}/*white*/
};

The clock code is trivial, especially because it's using the great Neopixel library. Below is the most important function:

void displayTime()
{
  byte digit1 = hour/10;
  byte digit2 = hour%10;
  
  strip.setPixelColor(0, strip.Color(color[digit1][0], color[digit1][1], color[digit1][2]));
  strip.setPixelColor(1, strip.Color(color[digit2][0], color[digit2][1], color[digit2][2]));

  digit1 = minute/10;
  digit2 = minute%10;

  strip.setPixelColor(3, strip.Color(color[digit1][0], color[digit1][1], color[digit1][2]));
  strip.setPixelColor(4, strip.Color(color[digit2][0], color[digit2][1], color[digit2][2]));

  digit1 = second/10;
  digit2 = second%10;
  
  strip.setPixelColor(6, strip.Color(color[digit1][0], color[digit1][1], color[digit1][2]));
  strip.setPixelColor(7, strip.Color(color[digit2][0], color[digit2][1], color[digit2][2]));
    
  strip.show();
}

The video I took of the clock in action is, for the lack of a better word, of too little value :), mainly because my camera doesn't seem to distinguish the colors as well as my eyes.

Even though this photo sucks, I added it only as proof :)

The time is shown as "HH MM SS", with spaces between hours, minutes and seconds. 0 is represented by an unlit (off) pixel. It is easy to get used to the colors, with help from the changing seconds.

Note that the 595 shift register on the board is not used (could even be removed, or not mounted at all).

Secondly, here is how one can save $20 on WiFiChron with GPS: use the $12 ublox instead of the $32 GPSBee. Some minimal wiring is required though, to connect 3 lines (5V, Gnd and Rx) to the module. Luckily, the GPS module has the right dimensions to fit in the spot for XBee (without the 2 headers mounted, nor the 74HC125, which is not needed in this case anyway).

If you really want everything to fit in the Serpac A20 box, you need to replace the original antenna with a smaller one (but working just fine), as shown in the next photo.

Thirdly, Craig (thanks very much!) published on thingiverse the plans for a 3D-printed enclosure he designed for Wise Clock 4: