Hi Aryln , Basically if i need to set a Material to a Static Mesh i will be using a create Dynamic Material Instance for the Static mesh and set the Material there .
if The DynamicInstance is not working for you from Desgin Blueprint . Which should work basically .
Try This : In the construction Script of you BP_Box Chaos : create the DynamicInstance of the material and promote the material to a variable called MeshMat and set its properties as InstanceEditable and Expose on Spawn .
Fetch the Selected Material From UI : Spawn BP_BoxChaos using SpawnActorFromClass . you will see the the material variable MeshMat as input in SpawnActorFromClass Node , which you can provide the Selected Material From UI .