Kydex is great for many things, being sewable is not one of them. It's acrylic/pvc that is thermoformed then drilled and riveted. You could sew two pieces together by hand after you've drilled the holes if you want, but you're not going to run it through a sewing machine.
Your best bet is probably to check out a costuming/wardrobe or cosplay forum. Sewing foam will be easier than finding a plastic that is thick enough to be rigid, but thin enough that a machine can sew it. Alternatively, you can probably use some leather tools to punch holes in some thing PVC/ABS/Etc.
Either way, you're going to want an industrial machine, and most likely a walking foot shoe machine like a Singer 29. Try it on a little Walmart special and you're gonna smash the machine with a sledgehammer out of spite. It may be preferable to take whatever your design is, and take it to a cobbler. Most shoe repair places are more than happy to do little sewing jobs.
As for the xDeep stuff, what part are you specifically talking about? My Stealth has metal shoulder and lower back plates, nothing plastic. Also of note, Cordura is a brand name for a set of fabric technologies, not necessarily always nylon, and 1000d nylon is not necessarily Cordura brand.