Here is a downloadable template kindly supplied to me in hard format by Nathan Taylor.
Make checks yourself as to whether this fits with your own snorkel before cutting and drilling etc – the template fits with the make and instructions included below :
Downloads below : A template, and the instructions and a diagram and this pic
It prints onto 4 sheets of A4 – the sheets are intended to overlap, so line them up at the same place on the tape measure showing on it. The links open into a pdf.
I hope it helps folks