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.

Update (18-April-2015): I modified the script a little to reduce the height of the backplate and added a couple of variables to make it easier to tweak. I also updated it to work with customizer.

I printed this with the fingers on the bed. There is a lot of surface area if you print this with a large number of slots. I did 16 in my picture and I did get some curling on the edges. I had the bed heat off and did not put any glue down before I started though.