I built this to get my project specific cables out of a plastic bin, so the stl files may not be very useful, but I also include the OpenSCAD, so it might be a good place to start modifying it for different size and number of cables.

There are four parts to the cable clamp, and a template for drilling holes.

I designed the piece that goes inside the box use heat set inserts, but it wouldn't be hard to make the holes smaller and use nuts instead. (Perhaps glue the nuts in place so you don't need 3 hands to assemble - that's why I used the inserts).