Before punching the holes, I used a marker to mark the right spot. I used 5 mm holes, because they worked best with the belt buckles I used.
I used Prym "Sport Mini" press fasteners, which turned out to work really well and I had no problem attaching them to the rubber. In order to give them some extra strength I used two, but one should be enough. The fifth picture shows how I marked the spot where the fastener on the other side has to go. Just press the other one down, it will leave small dent.