The right idea but too much work. Just tape a length of paper, butcher paper, wrapping paper, whatever, across the bottom [or the top or both depending]. Trace the curve and sides with a sharpie. Tape the resulting template to a wall, put a post-it** on the template, take a picture.
Now comes the cool part. Import the photo into whatever software you're using. Draw a rectangle around the post-it. Map, power clip, whatever it's called on your software, the photo into the rectangle. Make the rectangle the actual size of the post-it. Unmap the image. Now you have a full size image of the template.
**post-it or framing square or whatever you have that's square and of known dimensions.