Cutting a mast slot

Hi, I' having to repair a 8m carbon mast that has had the slot for the sail cut too wide and poorly as in not straight vertically ( we are talking probably only a mm or so ). Plan is to reform the nose using 6 layers of glass over the nose and then sanding back the excess down the sides to reform and aid the existing slot.

Does anyone know how mast manufacturers cut the slot ( I presume its some sort of mounted grinder ) or has anyone any good ideas on how to cut the slot.

 

I used a spacer of the desired thickness that was pulled out afterwards. It's here if you scroll down: Gougeon 32 Refit in 2015: New rig and other parts. http://gougeon32.blogspot.com/2017/01/new-rig-and-other-parts.html