The one thing you can do to help RFI coming off the boards is to twist the +&- together both in and out of the stitcher board. Start the twist as close as you can all the way to the LED board. Likewise to the power switch on the input side. When I was working on reducing RFI on some LED's I have, this has the single biggest effect on radiation coupling out the antenna.
And since you're obviously a decent solder person, some ultra short lead .01uf Ceramic caps across the in and out of the boards helps as well. But that might already be what C2 & C3 are for on the board.
Question: Why didn't you just buy 3000k 3W boards rather than building them?
