Around Annapolis, there is an easily accessed supply of marine plywood. This produce is about the most rigid and stable material available. It comes is many finishes - some with different hardwoods on each side. Although more expensive than others, this material will make the only enlarger base you'll ever need or want. A little polyurethane varnish and it'll last through almost anything you can do to damage it. It is easily worked and requires little finishing, even on the saw exposed edges. I would recommend 1" thick mat'l or at least 3/4".