Homemade View Finder - Take 3

I decided to try and solve one of my current frustrations.  A well-defined grid helps me draw.  Grid lines can go directly on a picture. With branching out into still lifes, it's back to using a view finder. I want a number of view finders, but don't want to spend a lot.

My first attempt to create grid on my homemade view finders was to use dental floss. This idea came from a blog. It worked for about 2 days, after which the floss got loose.  I also was worried about remember to always put them away, since I have string-eating cats.

A better solution was to get a cheap clear surface that easily attached to my homemade view finders. I found some clear (paper) sheet protectors and tested to make sure that lines drawn with a thin Sharpie marker would not smear.

It was easy to cut a piece slightly larger than the opening, tape it to the back, and draw grid lines on the front.

Here's the back.

Here's the front.

This may not be a perfect square, but it's pretty close. I'm using a pad of 3" by 3" post it notes to figure out the piece that I want to cut out.  If I want a smaller opening, it's easy to draw a smaller square inside the 3" one and cut that out.

I think this is going to work and I feel extremely clever!