WARNING: Doing this unproprely might lead to toxic fumes inhalation, so be safe and do this at your own risk.
Things you need:
-1mm sheet of polycarbonate (enough to make a few tries)
-A3 size sheet of paper
-Some thick scissors
-An electric hoven
Step 1: Shaping
-Fold the paper in half and align it with the center of the visor hole vertically.
-Trace the outline from the inside of the helmet. (Use some tape to keep the paper in place)
- Remove the paper and looking at your helmet decide where and how much do you want the visor to overlap over the helmet.
-Draw the new outline outside the previus one (remember it will have to reach the bolt holes from the MTB visor)
-Cut around the border and unfold the paper
Now you have a template to cut the polycarbonate sheet. Use some thick scissors for that.