The better answer is to move the drain line to line up. Or, you could make a traditional shower pan and tile it. I'm not a pro, but I can't think of a good way to fill thatopening and keep it waterproof, at least without spending a bunch. How's your fiberglass forming skils? You'd never get the finish to look like the existing walls, but you could bond that in. Very much of a hack, though. A picture would help. What exactly are the walls made of and how do they connect with the base?