You can take a heat gun and pliers and heat the edge and bend it up a little. Be sure to have the sheet clamped down so you don't warp it. Clamp to a table with a couple inches hanging over. Heat and bend that. It doesn't take much of a bend to keep the water in til it dries. Not sure how to do it if you wan the piece completely flat, you could maybe put some type of barrier to hold it in til it sets. Heres a pic of what it looks like when you do how I explained. Hope this helps.
