Here’s the design:
https://grabcad.com/library/fixturing-clamp-1
This chart has a great set of feeds and speeds to start with (6061 Al):
http://carbide3d.com/nomad/feedandspeed/
Use a 2 flute end mill, coated if you can, and you need to be really vigilant about chip clearing. A little lubricant can help (WD-40 is terrible stuff, get some real milling lube…but if it’s all you have, it’s better than nothing.)
This thread: Depth per pass calculation has a picture of the clearing setup I’m using with compressed air, works pretty well for me. The exact alloy you’re trying to cut makes a difference. I’ve had a lot of challenges myself getting a good finish, but it’s been fun getting things worked out.
The first couple clamps I made ended up with a lot of chatter, but it didn’t make a huge difference for the purposes of a clamp.
You’re going to break some tools getting things dialed in. Get a handful of cheap ones off ebay or amazon until you’re comfortable - I use these as my “throwaways” https://www.amazon.com/HQMaster-Carbide-Router-Acrylic-Hardwood/dp/B010NI39WO/ref=sr_1_1?ie=UTF8&qid=1473549793&sr=8-1&keywords=1%2F8+end+mill - they’re cheap so I don’t get too upset when they snapped off. They not coated but work well enough for roughing.