Heres how I did the seats for my COE.I marked out my pattern to cut with a sharpie. Then I used a large flexible grinder pad to do the cutting. You'll definately want to go slow and practice on a scrap piece first. If you have some way to screw the foam down, do it. I used a couple of nails to a piece of plywood.
The way to get a nice smooth edge is to repeatedly bump the grinder on/off. That way you dont get the grinder going to fast or youll send the foam flying, burn it, or cut a great big gouge in it. If you just need to make a cut, be very slow and steady and use the disc to cut along a line like you would with a saw cutting wood. It takes a little practice.
I also used a nice heavy duty piece of carpet pad on top of the foam, as it will hold up longer then just the pad itself.
Hope that helps. Good luck, Jeff
