you first option is to do it with hand tools using a plane. this would work but might take some time and effort. the second option is to take many passes with the cnc starting from the middle on one side and going all way down that side in small increments then continue on the other side. you would probably still have to even this out with a plane or sand it. in the picture it looks like it is plywood so you could bend it by putting water on one side. the wood whisperer has a great video about this. this is the best way i can explain it to you but do some digging in the web and on youtube, i am sure other people have had this predicament too.