Hi, since you are 3D-printing the case, you could have it printed from a translucent or thin white material and painted (JLCPCB offers spray painting now, others probably too), then use a sandpaper to remove paint from the feature you want to be lighted up. Works well for small batches, you just need to think about the brushing process (maybe prepare a metal stencil for the shape). SLS or similar can be a good diffuser for light.
Another cheap option is to make the front panel from a PCB and remove top+bottom copper and solder mask from the sections you want to light up.