I also used MDF. Two pieces of 3/4 laminated. I layed out where my miter track and t-track were going to be, and then after applying glue, I countersunk #8 screws on 6" spacing. You could also remove the screws after the glue has dried. I did this while the top rested on my tablesaw. It came out flat. Research suggested that plastic laminte on both sides was a good idea(supposedly resists warping/sagging). I did this and so far am very pleased with the result. I have also read about guys using baltic birch plywood, but have no experience to recommend or not to recommend it. Seems like it dosen't really matter what the material is as long as it stays flat. Have fun building your new top.