I pulled an old clamp out of my workbench and decided to model it in 3D and print it out, turned out pretty good.

Also the hardware I used is 1/4-20, there are a few knobs around you can print or can purchase at hardware stores or on amazon.

11/15/2017: Added a smaller version of the hold down I use on my miter saw, also added Fusion360 and Freecad files, Have Fun!