Always stuffed old 40-50# dog food bags with old clothes or plastic wrap. I buy bag of food every 2 weeks or so, save those bags as well to use inside. Eventually the "cover" wears out, so the next one is a new cover. Keeps everything inside pretty dry as well. Stuff, roll the top over, staple it together. Throw grommets in it to hang it if wanted.
Window screen makes a decent filler as well.
Had an old target shooter tell me to use old phone books. Stack them up, shoot into the open page side, not the spine.
My personal backstop is an 8X8 foot shed filled with old foam spa covers. Horse stall mats hang behind that, but never had an arrow get anywhere near it. Open the covers up, take out the aluminum spine that's in the center, and close it back up. Can always move the layers around a bit when one starts getting holes in it. Hung some old carpet in front. Leave my 3D targets in front, and the backstop catches any misses or pass-throughs.
Now, for distance markers. Shot in distances with laser rangefinder. Dug out and layed in bricks in 10 yard increments. Level with the dirt. Can mow right over them with no problems. 4-5 bricks side to side for each distance. Hit with weed killer 1-2 times a summer, and they are easy to find.
