I long piece of wood is all you need. Before. Now I have used a book or anything with a straight edge.

I've always used my seam quide for channels or repeating grids. It's that sliding bar that you attach (usually to the presser foot shaft) and quide the extending foot along a previous seam or sewing line. You only have to mark the first line in each direction. I've even used it for repeating circles in increasingly larger rounds.