I could not find a 18 degree wedge as large as I wanted, so I made my own. I used the smaller wedge I already had and just increased the length/width to suit my my needs. I used a clear acrylic plastic piece I already had and after drawing the lines, I cut it with a scroll saw because it was the only thing I had. The edges were a little wavy so I used 100 grit sand paper to get them straight.
Hope the wedge shows up. BTW way it is 15 inches long, 2 1/8 inches at the smaller end and 6 7/8 inches at the the wider end. I have some plastic left over, so now I'll make either a 10 degree or a 9 degree wedge.