Here are a few choices of guitar templates
that I would advise you consider.
1) 1/8″ Plexiglas: Plexiglas is clear, strong and stable. It is crystal clear, so you can see through it, and you can make nice clear registration lines on its surface. Even though Plexiglas is rather strong, it is also quite brittle and if you drop it on a hard floor it will crack if it hits just wrong.
2) 1/8″ Lexan: This is my favorite template material. Lexan will not break or crack. (Note when bullet proof glazing is mandatory, Lexan is the first choice in products). Be aware of that Lexan can be a bit stubborn to work. The consistence of the product is that it exists in a semi-liquid state. You can tell this when you cut it as the blade melts it. To avoid this, cut the material very slowly. Lexan, because it is soft, does scratch fairly quickly too. Obtain the clear Lexan so you have very good see-through properties. You will appreciate this when positioning your templates on the work piece.