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.
Accute matte screens are covered by a sheet of clear glass where you can make your own marks e.g. with a sharpie or with small pieces of clear or matte adhesive tape in the cornes. Both are easy to remove and a drop of alcohol on a piece of cotton wool will take car of any residues.
Accute matte screens are covered by a sheet of clear glass where you can make your own marks e.g. with a sharpie or with small pieces of clear or matte adhesive tape in the cornes. Both are easy to remove and a drop of alcohol on a piece of cotton wool will take car of any residues.