i have seen two nails put on the bottom outside of the curve, then you get a peice of one eigth or quarter inch lath and push it up in the center to the center of the top of arch required. and it gives a decent half circle shape.
What you are refering to is a semi-ellipse. There are several ways to create these. Probably the easiest is a mechanical method using a pencil, two nails, and a string. The nails are set on the horizontal axis of your wall or plywood, say 3 feet apart. The string is cut to four feet (plus a bit more to tie it off). Tie one end of the string to the first nail and the other end to the second nail. Now place the pencil against the string and pull it taught (like a bow and arrow except the pencil is perpendicular). While the string is always taught, draw a line letting the string drag around the pencil. This will give you an ellipse that is 48 inches by 32 inches (or a semi-ellipse 48 x 16). If you shorten the string, the ellipse will become proportionally flatter. If you lengthen the string, the ellipse will become proportionally more round. You will have to play with the spacing between the nails and the string length to get the shape and size you want.