I'd prefer the boxes probably. Plywood works and glue the joins so there is no air leak. Best stuffing is synthetic they use for upholstery. Don't use glass wool insulation. Be careful of the compass when installing them. If you haven't yet bought the speakers there are some good box speakers available that would save the hassle of building. Here's a couple of pics showing what I did on a previous boat.
Living aboard in Victoria Harbour