Customizable antenna wire clamp

I needed a sturdy antenna wire clamp and wasn't happy about the customization possibilities on the existing models, so I made my own.

3 screws, 2 cable guides, 3 nubs to keep the whole thing secured in place.

```
// customizable parameters, all in mm

LENGTH = 36; // length of clamp
WIDTH = 20; // width
HEIGHT = 4; // height
SCREW_DIAMETER = 3.2; // screw diameter
BASE_REDUCE = 0.85; // how narrow the base should be compared to the main size
CABLE_DIAMETER = 2.7; // cable diameter
$fn = 30; // rendering quality, at least 20 for the STL
```