Putting electronics in sand to keep them cool?
Sand is a pretty good thermal conductor (k=0.2) compared to air (k=0.02)and good clean silica sand is an excellent electrical insulator.  Vegetable Oil or mineral oil is about the same used for transformer cooling (k=0.3)  . and only has a dielectric constant of 2... but messy if your case leaks.

But forced air can improve thermal resistance by 60% or more depending on surface area+ roughness with just a low velocity of 3m/s just over a large flat area or a heatsink. (my own test results)

The limitation of heat transfer could possible be to the outside surface area for cooling the sand thru the case but it would do well in spreading the heat.

As an example of heat transfer, I recall seeing a youtube video of middle eastern coffee brewed with extra fine coffee in a small BBQ container filled with sand.   The aluminum pot for   a few small cups takes several minutes on a 2kW stove top. 

But when the small thin pot is dipped in propane heated sand pit, it takes only one half a second until it foams up., which is great for mass production at an expresso party.

Although it might be worth trying, I would use fins with a 30 mm fan. :)