oneproduct: can you float the idea of a rubberized sheet of some kind? some kind of durable, multiple-layer deal maybe a bit like an ESD mat, with some ability to compress to accommodate the underside of the pcb, but not so much that the pcb can flex. thickness would be maybe 1-2mm.
also, materials-wise we want something that won't build up static charge.
removable or bonded to the metal would both be fine, as long as it was precisely cut to fit.