I found something similar to overhead transparencies (remember those? From before the days of laptops and projectors). They worked great! Easy to cut with an exacto knife, you can see through them (that's what I didn't like about the cardboard option - can't see to position it correctly). Used a pounce to transfer the design - other than a few spots being hard to see, it worked really well.
I'm figuring out that marking a quilt is never going to be quick and easy - but I think I have an option that works pretty well now. Thanks for all the suggestions!