Indeed Epoxy/Sand (EG) is often used for machining bases (like CNC´s) etc to provide mass in order to reduce/eliminate vibration of it all. Which is also what we´d want for a speaker cabinet
You can see how I am gonna go about this on this pic, a test cabinet made with one single driver:
The EG will fill the entire cabinet, except for the volume the drivers themselves will use. Therefore the whole structure will have great mass, a rigid core (EG) which will prevent any sides of the cabinet to move/resonate.
Ideally you´d want a really high static vs moving mass ratio as this help eliminate structural resonance.
Image one driver and a card board cabinet. Ratio will be low, maybe 10:1
For this project I am getting close to 1400:1 which will practically make the structure immune to any forces excerted by the drivers. (Any force have a equal force in the opposing direction)
Each speaker volume will have batting etc.
Its gonna be a real handfull to move, but hopefully sound great!