I just use the grid marks on the Hassy's screen. Of course, if your screen doesn't have grid marks, that could be a problem.

For use as a composition guide, with the intention of cropping from the 6x6, as long as you get the size ratio reasonably close, you'd be OK with just a sheet of clear plastic (e.g. overhead projector slide material) marked with a Sharpie. Trim it to size to fit under your viewer of choice, and you'd be good to go.