Rick,
I used the standard X-quad file then just added four more motors and configured them. When you look at the std quad mixer, make the motors with the same direction of rotation have the same yaw values, the motors on the same sides (left or right) have the same roll values and all the fronts (or backs) have the same pitch value.
I was out flying my DW flat Okto today and it was windy where I was flying. As a result the motors had multiple cameo appearances rendering the video worthless. Guess what I was thinking about, yes, an XL-XY-8 with my good mount on it. Guess where I'll be tonight, yes, in the basement building an XL-XY-8!!
Why haven't I already done this?!?!?