Selleys "Kwik Grip" or similar "contact adhesive". Same as used to stick laminex bench tops down. Smear on both surfaces, wait for it to dry (10min ish) and press together.
5 min Araldite and just about any other waterproof glue will work though, but you will have to hold the pad down while it sets. That's where Kwik Grip is best, it will hold straight away if you let it dry on both surfaces *then* press it down for coupla seconds.