I use cardboard laminated together with carpenters glue. I had a gallon jug of it once , which lasted quite awhile . You can add water to the glue to make it go a little further . I usually made the backstop 1 3/4-2 inches thick , which would be about 7-8 plies . Precut the pieces that you need , have glue and paintbrush ready and have at it . Watering the glue helps with spreading . Once everything is glued you need to compress until glue sets . Setting on a flat surface and weighing down with any heavy objects will work , leave overnight for the glue to set. All of this needs to be done fairly quickly, as the glue sets fairly fast. After it has set overnight , it is ready to shoot . I usually ended up with edges not quite even , because of it being slapped together fairly fast . This can be fixed by trimming with a circular saw or table saw , and finishing the edge with duct tape. For that downtown look .
My bows were in the 40-50 # range . Now 40. Once the arrow is through , the broad head can be unscrewed , from backside and shaft pulled through . Or if you check for alignment with entry , broadheads like zwickey can be pulled through without unscrewing.
PS: Vinyl siding used to be shipped in cardboard boxes ,12’ in length. This is the cardboard I used .