Reading the above posts made me remember that when I painted my walls for a backdrop I wanted coved corners. I used a type of cardboard or illustration board I picked up at a craft store. My layout was high to make it easier to duck under (it was around the walls in a 10 x 12 room) I would guess the board was about 2 x 3 or 4. I sprayed the 2' edges with a spray adhesive by 3M. Just a band about 3" wide. I then held it against the two walls which formed the corner for a couple minutes. This provided a nicely coved corner. I used spackle to feather the edge of the board to the wall surface. This worked well for the two to three years before I moved. I should mention this was in a rented apartment. When i moved the hardest part was removing the spackle. Also some of the paper from the sheetrock pulled off when I removed the coved boards. So the spackle job I was faced with after removing the layout was formidable. Nonetheless, in two days the apartment was good as new, and the coved corners had made me happy for over two years, so I didn't feel too bad about it.