Ditto with Jan's suggestion. Screw a long 1 x 4" along the entire back. Set it a little down from the top of the current desk so when you swing the back up the edge of the drop leaf will cover the added mounting board and be level with the top of the desk. Do add legs to the leaf for support. Better yet- check thrift stores and watch Craigslist or Freecycle for a sewing cabinet, desk or table for conversion. You can find them very cheap or even free. Eleanor Burns has something on her website showing how her son converted a table years ago.