I recently saw where a person was using post-it-notes to level the horizon from one slide to the next.
I use a little program named Line Reader which I think is ideal for this situation.
The primary use for it is to follow the line of whatever you are reading but, it has extras that you can add different shapes to your screen such as circles, ellipse, and rectangles. Whatever you use stays on top on screen and you can move around as needed.
Below is the link to the program, which is $19.95 USD.