You could just make one from photoshop or something then import it into FCP X. Then drag it to the timeline over the top of your main storyline clips and extend it for as long as needed in time. Select the grid clip then go into inspector and set the opacity to make it as transparent as you need for a reference. Obviously it's not a FCP X thing, and I have no clue if such a feature is lurking in there somewhere ... but something like this would get you by. Best wishes.