You would have to take a existing cover... Split it into front, back and spine in a graphics program(e.g. photoshop,gimp,paint.net, etc.... , NOT mspaint in windows) then resize the front, back and spine individually to the right dimensions for a genesis case and finally put the three back together....
Voila.... If you know your ways in photoshop it's a work of 5-10 minutes.....