The first thing that comes to my mind is to I see about returning the unit and exchanging it for one that has a 2" drain opening. HD is usually very good about exchanges, and in this case at least, they sold something that does not conform to code. That would be a pretty strong argument if push came to shove. If that doesn't work, then I wonder if it would be possible to enlarge to hole to 2". In the real world, an 1-1/2" drain will probably drain the shower quite nicely, but one should stay within code.