You're thinking on the right track.
So the UHMWPE you can buy like as a cutting board has zero ballistic protection. It's some kinda fuckin wizard science to do with the structure of the molecules and how they are arranged, that UHMWPE ballistic plates stop bullets. With ordinary UHMWPE like with a cutting board, it's molecules are disordered, it's just plastic, it won't stop shit. Nylon is also pretty useless. Ballistic nylon just isn't.
You're on to something with the PEI 5 porcelain tiles though. A more proven design would be to copy cheapo plates like the Hesco 4401 which uses Alumina, polyurethane and a fiberboard backing.
So these PEI 5 tiles aren't very expensive but are very hard, not quite as hard as alumina though. Polyurethane is sold as truck bed liner. The fiberglass you'd need is the fiberglass cloth like used to make boat hulls that's pasted on in layers with export resin.
So, I would make an improvised plate with a tile, adhere it to a thin stainless plate as a crack arrestor. Idk if the corrugated side of the tile should face in or out. Then back it with about 16 layers of marine fiberglass cloth. This is a tedious pain in the ass process, BTW. Then coat the whole thing in truck bed liner. Might also add a thin layer of foam on the strike face and edges for drop protection. Wrap it up in nylon clothe so it's pretty.
I've been meaning to make a few plates like this and shooting them, but have never gotten around to it. I suspect they'd be thicker and heavier than a real plate also flat and not comfortable to wear but should stop pretty much any rifle bullet.
So they make fiberglass board designed to stop bullets. It's used in bullet-proof architecture like security guard boothes, saferooms for rich people and blast-resistant federal buildings and the suchlike. It might be possible to order a sheet and use that as a backing but idk.