Mmm..I dont know if i understand well what you want to do...But, cant you use some sort of plastic piece with a "L" profile, and use screws to fix it to the wood? I mean, for example, to hide cables in offices and so, sometimes some sort of square plastic box runs along the wall.If you cut that in two (not by the center of the box, but along 2 opposite vertices), you end up with 2 plastic pieces with a "L profile, thin enough so it'll still look like the screen is "carved" in the wood..Dont know if i explained it well, or it's not what you want, anyway :-D
Actually that would indeed work.... now to the hardware store to see if I can find something of the right thickness.... If I can than that would guarantee I am level across instead of trying to sand it perfectly!
If not I think I have an alternative in my head with your suggestion also!!!
Thanks