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.
Step 2: Bending
Polycarbonate melts at 155 deg celsius becomes gluey at 147 and deforms around 130. http://en.wikipedia.org/wiki/Polycarbonate
So put the hoven at something between 120 and 130 with ventilation on, wait till it reaches temperature then place the visor like in the pic..(align well the 2 sides)
Note: every so often it will start popping out, use a kitchen glove to put back in place
It souldn't take more then 7-10 min depending on the effective temperature inside. The best is you pull it out every now and then and check, the sooner it holds the better. Also is better to overbend it then under...
Step 3: Finishing
-If you havn't already done so sand the borders , they can be cutting sharp sometimes..
-Squash the visor on the helmet and pick the points to drill for the holes.
-Drill the holes apply using the original screws.
-Add stickers ..GO BOMBING!
Mine has gone through 2 years of abuse, so is all scratched, but visibility is still great, and there is no visual deformation whasoever..