Putting that Mechanical Engineering Education to use here...
Vaccum forming an existing visor may prove difficult for the following reason, I will try to not get to technical.
The polycarbonate (plastic) found in most motorcyle visors is fairly tough material. It must be able to withstand the high impact force of an object hitting the visor while being worn, and not shatter, resist heat and warp when warm, and resist becomming brittle at low temperatures. This material starts off as beads, and under high temperature and very high pressures injected into a precision mold to form a visor such as you can purchase. Most of the visors you can readily purchase gain the metallic reflective surface from a process called "flow coating" applied after the visor has been formed.
Attempting to vaccum form an existing visor wil most definatly destort the optics, may destroy the flow coated reflective surface, or at least cause it to discolor or form "stretch marks" as you are strecthing it thin, also, heating the visor to a point in which it "could be" vaccum formed may cause it to warp.
My reccomendation, use two visors, double layered as has allready been done, or find a local sign company that can print a clear or nearly clear (possibly gold if it matches the visor color) decal shaped like the raised surface to apply to the outside.
Hope that helps...
