Well, carpenters used to solve problems like these before electricity was invented. Then again, they had decades of experience and a shop full of tools Personally I don't think the Dremel will be of much use. What I would probably do is make a good, straight line where you want the cut. Then cut close to this line with your hand saw, doing the best you can. From there, start making the edge flatter and flatter, watching your line as you go. Start with 36 grit sandpaper, then finish with 60 or however smooth you want it.