@Davor recently posted a request for an alternative to the SunLED reverse-mount RGB LED we used on nearly all NeuroBytes boards. @Jarod White actually had the same thought when he first joined the company, and I created a 6mm x 6mm castellated daughterboard that allows one to reverse-mount a common-anode 0606 RGB LED (I used the LTST-C19HE1WT from Lite-On) in place of the specified device. I had to dig this one out of the archive; I only tested it on an old-logo-design ARM prototype:

The boards look a bit different but the LED effect is the same. And I included a sweet concentric circle silkscreen pattern on the back for good measure. I think I did forget a polarity alignment mark though.

Design files for the daughterboard (KiCad and Gerbers) are in the Files section. Uploading the design kicks back an error on OSHpark as the boards are just below 0.25" x 0.25", but it's simple enough to pattern the design in KiCad so several are fabricated at once. And now that the SunLED devices appear to be out of stock (at least on Digi-Key), this design may become ever more relevant.