Some 1-1.25" foam from ebay, cut to the shape of the plate bags, done, works great.
Off the top of my head the -S is SAPI small and I think PTS do Smalls in their foam plates if you want a pre-made solution, or generally anything foam or hollow plastic that's a small - though I'd stick to foam as sometimes if a plate of any solid material is a little too tall you can have exposed hook velcro on the inside that'll eat your shirts (though I don't own an LBX PC, but that's common).