My neighbor threw out this desk, so I bought it home, drew around sewing machine, cut out the hole, took another board and cut to fit, then drilled 4 holes, put in 6 in 1/4" bolts with spacers made from small pieces of plastic tubing the length I needed for my sewing machine to sit down in. When the bolts were tightened, they could only go so far because of the spacers. When doing this you just have to make sure you have space to plug in your machine after you put it in place.
Now all I have to do is raise the back table 2 inches to be even.