its not going to be cheap. I found this place
[Link: acousticgeometry.com]and this place:
[Link: acoustimac.com]the only problem is its "printed" so its going to look like a weird cheap piece of art. I spent hours on researching trying to conceal some speakers in wood. Decided on a highly specialized paintable acoustic cloth. Will finally see how well this will turn out in a few months.
Unless your speakers that are already in the wall are expensive you might be better off just using something premade from monitor/artcoustics.