I have been experimenting with an idea. I know that some types of plastic are pretty easy to work with. The cloudy plastic jugs that are used for milk jugs here in the USA are pretty malleable... When I watched a video of how kydex sheaths are made, I said to myself, "I bet other kinds of plastic could be used for that." I was right. This is just a preliminary experiment. I bet some of the super talented dudes on this site will perfect better methods.... so here's the basic idea (hope you like it).
This is the kind of plastic you need.
This is what you can make with it.
First, you have to clamp it as tight around the knife as you can.
Then add heat. I used the heat gun shown, but you can also put the plastic in boiling water for a while and then clamp.
Let it cool and add grommets. And this is what you get.
It stays in pretty good. Here it's set up to be a necker.