Ren, it's a bit complicated but here's how I did mine. First, it's back to geometry class. Figure circumference. C=2 times pi (3.14) times the radius (length of your flat sides. Divide circumference by 4 to get length of arc you're trying to cover. That will be length of wood to cover your front curve. I used 1/8 inch birch plywood. Set table saw at approx 1/16 cutting depth. Make relief cuts on back every 3/4 of an inch or so. Took some experimentation. This first layer almost broke but not quite. It looks rough but then you take a second pice of birch plywood and do the same thing. BUT remember, you've increased your radius by at least 1/8 of an inch so, your second piece will be a little longer than the first. I was able to get second piece to curve w/ out making relief cuts. Go slow, be patient and clamp and glue at least every 3-4 inches between the two plywood sheets. I also found it was necessary to add some supports inside the arch to keep arc correct size. I can send you photos if you'd like (nicholas@wcc.net), Fester