https://github.com/AztechMC/Modern-Industrialization/blob/master/docs/ADDING_MACHINES.md#adding-new-casing-types

In the adding new casing types, the path needs to be:
modern_industrialization:textures/block/casings/<casing name>/{top,side,bottom}.png

not:
modern_industrialization:textures/casing/<casing name>/{top,side,bottom}.png.

Also incorrect here:
https://github.com/AztechMC/Modern-Industrialization/blob/master/docs/MACHINE_MODELS.md#casing-models