What is the best metal for a handmade/DIY small heat-sink?

The part you are trying to pull the heat out of is limited by its heat sync connections. So if you want to get heat out its important that those tabs fasten to some substantial aluminum material as close to the part as you can get. Remember that you have to get the heat out before it can be dissipated. Good mechanical contact with those wings with some kind of clamp and of course heat sink compound is needed to improve anything. 10 watts is not that much, but pushing 10 watts out of those small wings on that part will be difficult.