I have a "handy" tool that I purchased - clear plastic with lines on it, but I like your idea better - it's easier to put on the machine and has a longer line - my plastic works well for smaller squares, but the larger ones are harder since the piece of plastic isn't long enough. As a matter of fact, the last time I used mine, I did extend the line with painter's tape - never thought of just using that and leaving the plastic in the box - now I know.
Blue painter's tape is a good friend for quilter's - I keep a roll in my sewing box.