Boxes
You might try one of the local gun shops if you only need a couple. The downside is that these usually have advertising on them indicating a gun is inside.
Go to an appliance store, and get a couple of large sized cardboard cartons and then buy a couple of rolls of duct tape. Measure out the size you need, allowing for the ends. Take a couple of 2x4s, clamp them together where you want to make a corner, and bend the cardboard 90 degrees.
I usually like to double box rifles. The inside box is measured out, and the lines I draw are then cut most of the way through with a razor knife. Then I bend the cardboard backwards away from the lines, and then tape the edges with duct tape. I then make a larger outer box, but do not score the lines with the knife on this one. Duct tape the ends well, duct tape the seam, and wrap duct tape around the box at several places (about 5 does well).
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