They are on the right track. A gal in our quilt guild used a pizza pan for hers because it was the right size.
HOWEVER (smiles) If Murphy is a neat-nick and likes her sewing space "just so", she may feel more comfortable with a ruler that can be displayed or stowed away neatly without having to go about the house searching for just the right thing.
Murphy, it's really up to you, but for me....I'd save the money and work with something on hand. And I'd tell Everyone...Hey! Did you know my crock pot lid is Perfect for >......."THIS"!?
Good luck