I like that idea of using the paper or even sandpaper. I wonder how good an index card would be? I saw an HST trimmer by New Leaf Stitches
https://www.amazon.com/Trimmers-New-.../dp/B06XGKVC5J
Has anyone tried this? It's pricey, but looks like it would save a heck of a lot of time. I'm wondering if I can figure out how to use one of the triangle rulers I already have. The only thing I will be missing is the slot to cut off the dog ears.