My old long board was falling apart and was not stiff enough so I made a new one today. Another problem was the sandpaper strips were peeling off of the ends from flex in the board. Initial test today was good. I used a 3 ply sheet of 1/8” plywood and covered it with one layer of 200 gram carbon uni on one face. This made a stiff board but it had enough flex in it to conform to the shape and spring back straight after I was done. I made it wide enough for three strips of standard air file paper, tacky backed it comes in a roll.