Is there a way to use an alternate model for shadow rendering?

One of my game elements is a cube constructed of many smaller, more complex parts (wires, gears, rods, etc). The shadow rendering cost is a bit high, but ultimately the shadow shape is just a cube. I’d like to be able to turn off shadows for all my small complex parts, and use a simple 12-poly cube for the shadow rendering…

Is this possible?

As usual, found the answer right after posting the question.

Solution is here: