In researching the best metals for heatsinks, it became evident that after a certain point the material costs outweigh the gain in cooling power. At the top of the list of thermal conductivities you have Diamond and carbon nanotubes and cubic-boron nitride, which are leaps and bounds ahead of any metal in their capacity to conduct heat. The etymology of calling diamonds ice, rather than being derived from some rap artist, is due to the ability to draw heat. If you need to test(not used for any real analytical accuracy) a stone to tell if it's a genuine diamond, if you place the stone of your tongue a diamond feels cold, whereas most other crystalline solids will feel warm. This happens to be a rare trait in minerals, which is why many diamond testing tools use a probe touching the stone's surface to apply heat and measure how fast it dissipates.
Maybe in the future, with the advent of better diamond production techniques, diamonds may become common options for cooling electronic components.
The most commander heatsinks are either copper, or more likely, aluminum such as the radiator in your car. Aluminum has a conductivity of about 200 watts per meter per kelvin(WmK), and copper around 300. Silver has a rating of slightly over 400 WmK, making it the best metal that's easy to work with. This is why I decided to make my own heatsinks in silver, currently for cooling stepper motor drivers.
Step 1: Mold Forming
Any hoo, I used my milling machine to cut out a rough cavity about 5x5x20mm. After machining, use a piece of paper towel or rag that has been dipped into oil(any kind) and light it on fire, creating a sooty flame. Use the flame to deposit soot(unburned carbon) into the cavity of the mold, this helps to keep the casting from sticking in the mold(as this is a single-piece mold).
Now for step 2.
Step 2: Casting
For small things I use what's called a melting dish, a small silica ceramic bowl with a spout, to melt my silver.
For this all you need to do is melt and pour.
Step 3: Machining
Then use the same saw to cut perpendicular to the first set of fins, to maximize the surface area because more surface = more cooling.
Step 4: Finished Product
To mount the heatsink I bought a thermal pad on eBay, needless to say it didn't stick like I thought it would so I have since swapped it for thermal paste, which though messy worked better.
In the photos are the sinks mounted to a stepper driver IC.