I have good results with mixing aluminiumoxid with epoxy. This is usually good enough for potting electronics but not enough for high power LEDs.
All you need is aluminiumoxid with different particle sizes, long curing epoxy with very low viscosity and low hardness.
mix the compenents well with a "mass-percent" of 65%-70% Alumiumoxide, don't use volume units such as Liter.
The more Aluminiumoxide the better. If you have an epoxy with very low viscosity then you can even use up to 75%.
The end result can be compared with the surface of a washbasin. Very smooth and hard with good heat transfer.
I'm using this to cool the 9A driver otherwise the current sense resistor will change it's value since it has just a few milliohms.
I'm mixing each part of the epoxy seperately with aluminiumoxide and put them into syringe tubes and store these tubes in the fridge.
When I need them I just take the tubes out of the fridge and throw them into warm water. Warmed up the right amount of each part can be pushed out of the syringe tube and mixed up for use.
This stuff can be used for a lot of other things as its mechanical properties are very good.
And it's cheap