Since the SK6805 RGB LEDs are side-firing, they don't emit much light through the PCB, leaving a lackluster showing.

My first attempt at diffusion was to laser-cut some clear acrylic in the outlines of the shapes to act as a kind of light pipe in the hope that helped. The result was barely imperceptible.

I roughed up the back of the acrylic in the hope that would bounce more light through the front, but the majority of the light still was strongest at the edges.

Next I tried 3D printing a frame and adding reflective material to the back to help bounce the light through. This worked okay, but logistically it's a bunch of extra fiddly work and I still hadn't come up with a good way of fixing it to the PCB.

Finally I tried some hot glue, and it made the biggest difference for the least amount of work. It's a bit messy, but it works, and it's on the back where no-one is looking :)