For the packing problem, researchers are looking at "so-called REGULAR tetrahedrons. By definition a regular tetrahedron has four edges of equal lengths.
In other words, researchers are not trying to find new shapes of regular tetrahedra (I wonder who can), instead, trying new ways to arrange them in order to increase packing density.
If you found a new packing method that beats the tweeked Cornell Structure of 85.55 % packing density, congradulations! But beat out the God, LOL!