You could add a board like this Neokey Ortho Snap-Apart available from Adafruit (5157). I used that in my homebuilt big screen V5.1 acrylic.
If you follow the arrows with the LED chain (LEDs on the user-side of the keyswitches), then the LED snake/chain is identical to the one in V5 and compatible. Then you’ll need to solder a wire from each switch to the contact on the control board and from each row to GND and one 5V wire of power supply for the LEDs. If you need small diodes, you could take them off the Neo Key.
Ah yeah, and the LED data wire to the DI on the top row.