That's beautiful wood - love the contrasting grain stripes. It'd be easy enough to glue two of those boards together and you'd have plenty to make a good size box. Just plain one edge on each board so you get a true flush fit; center a saw kerf down the length of each; cut a long spline as long as the boards (or several shorter pieces work just as well); glue the splines in and clamp the boards together. Might need a little planing or sanding to flush the surfaces but you'd have boards 7-8 inches wide.
And that's some gorgeous koa Mike.