Sandpaper Cutting Jig
The hacksaw blade is shimmed up with very thin pieces of wood just enough that you can easily slide coarse sandpaper underneath the blade. The teeth are pointed out, or to the right in the first photo.
The first line is drawn 4.5" away from the cutting edge of the blade, and is used to place the sandpaper so you can tear it into two parts longwise.
The third line is drawn perpendicular to the blade so I can easily tear off odd-sized rectangles while keeping the cut square to the sheet. If I'm only doing a small amount of touch-up work, I'll only tear off a gum stick sized piece. If I later pick up the odd-sized leftover and want to use it in my palm sander, all I need to do is line it up with the 5.5" line and tear off whatever's left.
The pencil lines aren't ideal, and tend to wear off after heavy use. Making more permanent markings with a deep cut + black ink or burning the lines with a soldering iron or wood burning pencil would be a possible improvement if you make one yourself.