I'm building a jewelry box and managed to mess up joining the sides. After some careful planing, I have the top (the open part) perfectly flat, but the bottom wobbles. I've tried hand planing and sanding, but I can't flatten out the bottom. When I try planing, I chip off small pieces at the miter joints, so I don't want to continue that effort. If I had a drum sander, I'd pop it in top-down and I guess I'd get there. I'd appreciate advice! Thanks.