Making Your Own Wood Cornice
Although these instructions are for a wooden flower cornice, you could easily make this fit almost any type of decorating theme. It's basically a wooden box that's embellished.
You could make the cutout any shape you want instead of a flower. You don't even need to use a shape at all. You could just leave it box shaped and then embellish the front of the wood cornice any way you'd like. But the 3-D effect of the flower petals or some other shape overhanging the edges is really a nice touch of custom.
This would be really easy with circles, squares or triangles hanging over the edge. Leaf or tree shapes, car tires, all kinds of things. Look in color books or at stained glass patterns to get some ideas of easy shapes to do.
You won't want anything too intricate (tiny or skinny) to be in the over hanging parts, because they would be difficult to cut out and could be easily broken off.
Wooden Flower Cornice:
1. Make a newspaper pattern of the size and shape flower you want and use it to draw the scallops on the face of the cornice piece. Cut plywood pieces to the size you need for your window using the sketch above as a guide. Be sure to include the flower petals that hang over the edge on the top and bottom of face making a scallop. (This gives a 3-D effect)
2. Glue and nail or screw dust board and returns to the face making all edges flush. Note that these pieces are attached to the back of the face, so the edges don't show from the front.
3. Patch any holes with a wood putty, let dry and lightly sand the whole thing.
4. Use an acrylic stain blocking primer such as Kilz (my personal favorite) to prime the whole thing inside and out. If you need to, apply a second coat on the inside so it looks nice and white.
Paint the face of the wooden flower cornice, dust board and returns with your base color in a semi-gloss or satin finish, being sure to get the bottom edges as well. You may want to use two coats for the best coverage. Let dry overnight.
Using your flower pattern, lay it on the face of the cornice matching the scallops you cut, and trace it with a chalk pencil or trace it lightly with a regular pencil.
Use a small brush or sponge and paint in your outline with purple or your color of choice, then fill in the rest of the flower including the bottom edge of the petals. Now draw a circle for the center and paint it light gold, or whatever color will coordinate with your room.
5. To mount the wood cornice, use angle brackets of the appropriate size (based on the depth of your returns) that you purchase from your local home improvement store. If they aren't already white, give them a couple coats of your white primer.
Mount the brackets (2 for a cornice up to 48") with the appropriate screws or anchors as needed to the wall. On the 48" cornice above, try to mount the brackets 6-8 in. from each end, and around 8-12 in. above the window trim.
Now set the wooden flower cornice on the brackets and screw up through the brackets into the dustboard. DO NOT just set it on the brackets without attaching it!
If you don't have room to get up in there with a screwdriver, try predrilling the holes in the dustboard, and using your hand, push a bolt up through to the top, and fasten with a nut.