I think you would be better off cutting the center out, before you shoot it out. Run a 2 X 4 across the bottom, and the top of the OSB. I see a dowel laying on the ground for the roll type system. Maybe figure out a way to hook that up, protected inside a metal box, or guard on the bottom, that you could just pull up a new sheet, and cut it off. Metal guides on the sides, and a removable clip on top. Just some suggestions to make it easier. Wind may cause some problems, but I know what the wind does when you are trying to connect up a piece of paper also.
The one we made is wider and has a box enclosure that the rosin paper sits in. The top of the box has hinges so the paper is held taunt by the weight of the top after the paper is clipped to the upper 2x4. Works well for our needs Joe
Thanks for the replies fellas. I didn't think out the spacing for the paper holder. The width is 36" which is also the same width as the paper so it was a little too tight of a fit. I think building a box for the past is going to need done. Gonna work on it some more when I have the time to come up with something to hold it. Took it up to the gun club to pattern my buddies gun tonight and I gotta say, the OSB held up way better than I was expecting none of the shot even passed through. I have an extra backer for it so I think I'll just keep it for replacement down the road if I need it. @Stl Flyn i thought about cutting the middle out while I was building it but I wound up keeping it in for the added weight since it gets pretty windy at our club. I drilled holes in the base to stake it down and they held great so cutting it out is a possibility now. @eightbore haha. The next door neighbor was interested in it. I had to mooch a pair of clamps off of him because mine wouldn't fit over the 3 stacked 2x4s in the middle of the base.
Egledhill, If you go to the search forums above and type in "patterning boards" then type in Neil Winston in the poster heading and search shooting related threads Neil's design for a patterning board will come up about half way down the searched threads. This is the design we followed. Works great. Joe