The right thing to do would have been in install a lead bend under the slab or have the elbow down far enough that a piece of cast iron pipe was coming up. It doesn't sound right that the elbow itself is exposed. You're sure you have a cast iron elbow sticking up and not a piece of lead right? If you could get a couple pictures of the situation it would help.
What you have right now is a hack job, and using another piece of plywood would be another hack job. Someone else will come along with a more solid answer but you may need to have a plumber come and pour a new flange into place or you may be able to use one of those internal compression flanges.