Try this,
get a piece of flat bar, grind to match fender radius, and be square.
draw a line about 3/8 or 1/4 from fender edge with morphys or dividers.
start in the middle hold fender firmly and start to tap over the 3/8 part till it is about 90 degrees, then move flat bar or and repeat.
when you get into the sides (after middle is complete), regrind and square up the flat bar (1in or 1.25 should work here), and finish corners to 90 degrees.
when the whole cut is at 90, flip fender, put on a wood block, and gently hammer the edge down to flat..
good luck....