Sundance

Many woodworker supplies catalogs list carbon paper in extra large size for tracing out project plans.

I would start with a hole punch, 3/16" to 1/4" diameter, then follow with a knife or chisel cut to the slot. Look at a button hole in a thick coat to get the idea. The chisel will make a more accurate cut than the knife. It avoids the possibility of a slip with the sharp knife, making your slot too long. Sundance