Gday,
ESP & outback_ute are right on the money.
Some time back I got hold of a second hand shaker, to put on my ZH Fairlane. It did not come with a template for cutting the bonnet (and it would have been for an XY anyway). I had put a shaker on an XB years ago, and I did the ZH the same way:
-bolt the shaker on the carby.
-put masking tape on the front guards/fenders along the edge where the bonnet closes adjacent to the shaker scoop, and along the edge of the vent panel, outback_ute called it the scuttle panel, behind the shaker.
-make pencil marks, on the tape, every inch or so.
-take measurements, at each tape mark, being careful to try to keep square to the centreline of the car, from the shaker to the fender, and shaker to vent panel.
-write the measurement next to each pencil mark.
-remove shaker assembly & close bonnet.
-with the tape on the fenders you should have a pretty good idea of where the shaker will end up on the bonnet skin. Apply masking tape to the whole shaker area. (maybe that should read the shaker hole area!!)
-using the measurements you recorded next to your pencil marks, on the fenders and vent panel, make new marks on the tape on the bonnet to locate the cut for the shaker. If you got one of the shaker bonnet hole edge moldings, with the shaker, then you can use this to transfer the shape of the hole to your bonnet, in conjunction with your measurements.
-lift the bonnet, cover the carby inlet & set the bonnet down on a block of wood, at the front, so the bonnet is raised a small amount.
-drill a hole large enough to poke a jigsaw blade through, inside the shape you drew for the shaker, near a corner.
-cut along your marked line. BE aware of any bonnet strengthening ribs under the cut area and trim as necessary. You will, almost certainly, go through a few blades, so expect that.
-temporarily reinstall shaker & GENTLY lower the bonnet, taking very close notice of it touching the shaker ANYWHERE, if it looks like it will ,STOP and use a file, or grinder (VERY gently) etc. to gain the needed clearance. Also constantly check with the hole edge molding in place, if you are going to use it; a lot of people dont use them because they are designed for XY bonnets.
-when you are happy with the fit, prime & paint the cut edge. If the bonnet paint is pretty reasonable you can carefully use a brush to do this, as the shaker hole edge molding will cover it up. If you dont want to use the supplied molding, there are plenty of plastic & rubber trimming available that can look good, that is what I did on my old XB.
-then the most important step: drive it and enjoy it!!
Good Luck.