One option would be to go to your local hardware store and find a diameter of PVC pipe or even metal tubing that will snugly encase your sword. Then you could go about decorating the sheath with a simple paint job or if you would prefer, covering it with a wrap or even craft foam. Though the craft foam may end up being a bit more trouble than required. I'm not intimately familiar with the sheath's look and a quick search didn't find many good references.
All told, assuming the sheath is rather simply decorated, you may be able to complete the build with $10 US.
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