I have to disagree about the recommendation to try PP or PE with flame treatment and specialized epoxy. Flame treatment introduces oxide and hydroxyl groups into the surface which can interact with the epoxy, but the bond strength will never approach that achievable with high surface energy plastics. The bond strenghs achieved with flame or corona treated PP and PE are an order of magnitude lower than high surface energy plastics like nylon or polyurethane.
Here is a good surface energy table for common plastics:
Higher surface energy results in higher bond strength, in general. Nylon, polycarbonate, epoxy, ABS, acrylic, and polyurethane are at the high end (>40 dynes/cm).
I agree with Roger's suggestion to consider polyurethane. Some polyurethane resins foam when they cure (Gorilla Glue is one of these), which means you could produce a relatively light foam block strongly bonded around the valve, cut it to the desired shape, and glue it into the pad with polyurethane glue. The heat sealable inner surface of your synmat is polyurethane, and the glue that Thermarest sells for replacing their valves is polyurethane. The Hysol glue that many people use for bonding cuben is also polyurethane.