I really liked simple geometrical forms, so I got square 40W 4000K LED fixtures. They were relatively inexpensive (~40$ each), their large area ensures decent cooling and long life. I disassembled one to figure out if it's possible to stick LEDs inside - but there is just too little space left for high-power LEDs.

Fixture design is quite clever, like on LED display back light: There is acrylic light-guide with diffuse dots across the area to make it uniformly lit. light-guide is lit from the side.

After fixing minor issues (like black marker marking on the edge,tighter assembly to improve thermal resistance) I assembled it back. It was clear I can only add LED on the back, facing to the ceiling. That might be interesting effect though. I glued first LEDs using ALSIL-5 glue and generic "heatsink plaster".

For first revision I decided to use step-up regulators with ballast resistors to control the current over LEDs. I set the current in "hot" state. Surely in the future I will need to replace it with digitally controllable constant current sources.

First tests shows that with 5x cyan alone spectrum is very far from desired. I needed more LEDs, so I added RGB ones, intending to use green and blue dies.

Current adjustment near the ceiling feels a bit tricky:

With green on I've got the following spectra: