Didn't find a template but did find a tutorial
. Here's another tutorial
that shows the template they used. Here are some WIP photos
that can be replicated. Here's some line art
you could use for reference.
And here's a whole final fantasy tutorial collection
for anyone who stumbles into this thread later.
Not sure if that helps. I'm not sure what a template means to a commissioner. From what I've seen it seems like blowing the picture up to be the size that you want?
All of these are from the first page of googling "lightning farrons gunblade". I'm sure page 2 has more if you care to look.