I use 1/8 inch plastic found in any hardware store. Start with a standard scale and trace onto the plastic. Make the cutout slightly bigger. If you use a "sharpie" pen, the extra width of the tip is usually plenty if you follow the outside edge.

Put the blade over the top of the new template and make adjustments with the marker. Use the dremel to sand to the profile you choose. Put the blade underneath the clear plastic to see if it works and pick a spot for the pivot hole. Drill that one. You can mark for a spacer pin, but I don't drill that one in the plastic because that changes from blade to blade.

I have several different patterns that are slightly oversized. Once I match the template with a blade and cut out the material, I grind everything away that doesn't belong there... Basically, I have three patterns.