This rack consists of a custom number of 4mm and 8mm slots. The spacing between the slots is 8mm. The holes are automatically placed on the outside edges but if the number of slots is small they will be placed vertically instead of horizontally. In the STL I posted I used 8 slots of each size.
I'm planning on modifying this design to put a larger curve between the back plate and the fingers, to give it more strength, but it has no problem holding cables as is. I'm also planning on reducing the height of the back plate and possibly increasing the size of the slots.