OK, I will try to take a shot at describing what I have done for a window that large, actually larger. I used small, about six inch shelf angles screwed to the wall in about four locations that will be the height of the valence. The valence itself is a simple board or plywood long enough to go the full length. I place the board flat on top of the angles and drill a hole at each angle so I drop a short nail from the top through the board and angle, keeps the board in place in case it is bumped but you just take out the nails and the valence comes down easy for cleaning, etc. Then take the board down and attach end pieces and a front piece to the top board and end pieces. Your wife wants oak, maybe oak faced plywood if you can find it would be light, otherwise you may have to have some oak cut just for this project. In my case, I used plywood for the whole thing and covered it with foam and fabric. If you want to oak to show and look nice you may want to find quarter sawn material for the front and two ends. Good luck, GCS