I intend to do the same thing. How do you intend to build it? I have seen a a product that installs to the flat part of the ceiling at the bottom of tray. The drywall finishes to it and then you attach the cove or crown moulding to it to create the lighting cove. The only problem is, I can't find the product. Thought maybe you could help.

Don't remember seeing what you're describing.

I'm using 5" crown moulding installed leaning into the tray, 1/4" up from bottom of tray bottom edge for a reveal. Will look better than flush install, is easier to fill gaps (have white painted trim) and adds more interest with another shadow line. I've decided on mounting a cleat around entire tray to attach crown. This should allow for nailing crown anywhere and also provides a horizontal surface to mount rope lighting to. Already installed switched receptacle in tray where will be hidden by crown,

I haven't yet mounted crown with all the must-do things on my to-do list, but will post again to let you know results. Good luck with your project.