I made one for a kids party a few years ago. I asked the appliance store for a refrigerator box, and they saved one for me.
You lay it on its side. Cut down the middle of each end from top to bottom, slash a bit along the bottom of the sides, and fold the ends into peaks like a prow of a boat. You could leave the back end square. With an empty roll from new carpet, you can stand it up with tape and use heavy cord or light rope to support it on the sides. This makes a mast. I didn't mess with a sail, but a sheet would be easy to rig up. I also cut a door in one side so they wouldn't break the sides down by climbing in and out. A pirate flag goes atop the mast, of course.
At the same party we had a treasure hunt with a map and clues. They found a box of gold covered chocolate coins. We also had rubber sword fights and I gave everyone an eye patch. I asked them to dress as pirates, and we had some pretty good outfits!
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