There may come a time when you break/lose or don't have a knife sheath and I know that kydex is one of the materials that I've heard of guys using. I work with a lot of plastics at work and although sintra is nice and light it is also fragile and doesn't have good sun properties. We started buying ABS for redoing tank bases and had a lot left over so I scavenged a piece. ABS is very rigid, in fact it can be difficult to cut. It glues well with abs glue and is okay in weight, about the same as plexiglass.
You will need to find some of the sheeting, plastic depot in gardena might have small scrap pieces, otherwise I saw this piece on amazonhttps://www.amazon.com/Install-Bay-89-00-9031-Plastic-8-Inch/dp/B0007WTF02/ref=sr_1_2?dchild=1&keywords=abs+sheet&qid=1602294525&sr=8-2
9 bucks for shipping for a 12x12 is hard to beat. You will need a piece that is as thick as your knife blade, otherwise you will have to double up the middle part which I will describe later.
You will need: ABS Glue (home depot has it in the plumbing section), ABS sheet pieces, your knife, small bungee or knife strap, drill (optional).
The first thing you want to do is lay your knife blade on the abs. Figure out roughly how big you want your entire sheath to be first, and draw an outline on the plastic. After that cut 3 pieces the exact same size and shape. A power jig saw with a sharp blade will work, utility knife will not as the material is too hard. You could possibly do this with a hacksaw or coping saw. You don't want to use a circular saw as abs is super stiff and will vibrate a lot during the cutting. Hold your breath as you cut, when plastics are heated up they often melt and become fumes and they are toxic.
Take one piece that you just cut and then trace the outline of your knife on it. Make it slightly larger all around the blade because you want to be able to get it out easily. Cut out this outline and then sandwich the piece between the other two pieces. See if your knife fits inside it, without binding, if it doesn't you might have to double up on the material for the middle.
In a well ventilated place, put some rubber gloves on (all abs, plexiglass, pvc glue is super toxic and causes cancers). Glue all three pieces together and put a brick or something on it to press it well, let dry about fifteen minutes. While it is drying cut two small rectangular pieces for your bungee or knife strap. Glue this to the back of the ABS sheath. Clamp that as well and let dry.
When the entire thing is dry, drill a small hole towards the bottom of your sheath, right where the knife tip ends, this is to drain water and should go all the way through all 3 pieces. Then drill out the holes in the rectangular pieces for your bungee.
Note that I make my knife sheaths pointy, that is so the extra bungee that I put through the handle can be used to secure the knife to the sheath. The bungee fits around my wrist and also is long enough to stretch and go around the pointy part.
As a side note, if you have regular knife sheaths and have lost the knife save them, you will be surprised how many different types of knives will fit that sheath.