My dad used to make these sheaths for kitchen knives he used in the field. It could be thrown in a bucket, stuck in your belt, whatever. Cheap, safe, customizable, washable.
He would heat the pipe with a heat gun then squish it flat around the knife. Leaving the bottom open for drainage. Keep heating and adjusting for some retention. Insanely simple, and brilliant at the same time.
Do it in outside space, if it ignites, it will make toxic fumes. Safety first!