Most art and craft shops sell large pieces of plastic mounting board. They have a dense foam inside, sandwiched between a couple of sheets of smooth white plastic and are very lightweight. I’ve used these before and they give of a nice soft reflection. You will need two of these plastic boards.Lay one board down flat in front of you in a landscape position and cut it in half. Now cut the top of both of these halves at an angle (as shown in pic 1). I simply measured 6” down each side to give me the angle required. Discard the two small pieces. Now place the large piece (the full sized one) in the middle of the two halves and butt them against each other so it looks like one large board.