I'm repainting my turret and changing out the bottom and side window trim from 6" to 4" with Hardie Board. The top trim, currently is plywood cut in an arch shape. My contractor is saying that he can't use Hardie Board because a sheet of that is only 1/4" thick and would be thinner than the Hardie trim he's using on the sides and bottom, which is think are .75". Anybody have any ideas or some other product he could use to cut the arch trim out of?