As there are varrying sizes to the different makes, I don't think that I've ever come accross any templates... on the other hand, I did respond in a thread a while back on how to make your own template.
If your searches turn up nothing, then drop me a PM and I'll hook you up with the required info and reference pictures.
What if you were to use WOF's helmet templates? I mean the front section. This clearly shows the T-Section so If you were to go 1/2" larger as a start, then trim from there?
Or wrap some card around the outside of your lid and mark in pencil from the inside and do the same process of enlarging it?
What I did was to place a piece of cardboard ( like the kind from a cereal box ) on the inside of the visor opening. Make sure that you've got the top edge against the inside of the dome ridge, as this will aid in positioning the final t-visor for instalation.
 
You just have to trace the edges of the cut out visor opening onto the card, then once removed, add an additional say 1/2 inch or 1cm. Cut it out then test fit and make any adjustments needed.
Once you're happy with the cardboard template, you'll need to decide how you're going to attach the visor. The template works well as a mounting guide as well, so you won't have the problems of the visor not lining up with the mounting bolts ( or t-nuts/chicago screws ).
You can now use the template to transfer the shape to your visor.