I just made some adjustments to the flexSandContainer.uasset to make the properties match the ones in the Flex video. You can grab them in the following commit:
/commit/60572d8eba2b04aa72964a93c987d1bdba17fdd0
It makes the behavior much closer, and the piling is generally better. The resulting angle of repose will be a complex function of many of the parameters mentioned in that commit, you would need to make some empirical measurements to make the correspondence.