Detail a shape for me

In the example I've broken down a basic cuboid using 20mm square section into it's one "key" piece which can be repeated 12 times to make the cuboid object.

I need the same doing for a dodecahedron.

Again making the edges from 20mm square section into it's one constituent piece which can be repeated 30 times to make the object. ***Obviously the angles need to be exact***

I'm not requiring that the whole object is built back up once the key piece is made in 3D.

I'm requesting a 3D model of a single key piece only in an .obj file and a separate image file of the angles and dimensions.

Apparently I can't upload an .obj file extension but I'm happy to send it via email if required.