3D model viewer with transparent plane below drawing a shadow

[dilldilldill]

I try to achieve a 3d model viewer with transparent background and a shadow below the model similar to this. I am using the 3d model viewer example as a starting point.

Now I am facing two problems basically.

The first problem is that I don't know how to have a transparent plane that still can receive shadows. I found a hint on how to create such a material by using this mat file:

material {
    name : "Invisible shadow plane",
    shadingModel : unlit,
    shadowMultiplier : true,
    blending : transparent
}

fragment {
    void material(inout MaterialInputs material) {
        prepareMaterial(material);
        // baseColor defines the color and opacity of the final shadow
        material.baseColor = vec4(0.0, 0.0, 0.0, 0.7);
    }
}

But when I try to use the resulting filamat file that I compiled using matc I keep getting the following error:

reason: the material was not built for the OpenGL backend

This is how I set up the bottom plane:

Material.builder()
        .setSource(getContext(), R.raw.transparent_floor)
        .build()
        .thenAccept(material -> {
    //...
    Node planeNode = new Node();
    ModelRenderable plane = ShapeFactory.makeCube(
        new Vector3(3f, 0.01f, 3f), planePos, material);
    plane.setShadowReceiver(true);
    plane.setShadowCaster(false);
    planeNode.setRenderable(plane);
    scene.addChild(planeNode);
});

The second problem is lighting. When I disable the indirect light of the environment (indirectLight.setIntensity(0f)) and place lights around my object I only achieve two things. Either the model and the plane appear greyish because of the dim light but the model casts a shadow, or when I light up the model properly either by a few lights that I set up or I set the indirect light intesity to a higher value, the shadow is barely visible at all.