Can't help with template, sorry. When you get one and measure up, don't forget that you need a little extra space to slide the battery off the mount, effectively 3cm or so added to length.
cut a piece of cardboard, 36cm L x 10cm H to see first if it would go inside the triangle.
you also need to make sure that the bottom end of the cardboard clears the chainring.
Best post some pictures of the cardboard in your bike frame, we'll check that it will work for you.