Are you practiced with acrylic work? To get a good seam with weldon 4, you have to have a perfect fit between the sides for a seamless bond. Any gap anywhere and weldon 4 will be insufficient for a water tight seam. Your better bet, especially if aesthetics arent terribly important, is to use weldon 16. It gives you a bit more room for error and you can treat it more like silicone (as opposed to the weldon 4, which is thinner than water).