I would be tempted to curve some thin aluminium. It wouldn't need to be more than 1.2mm. Covered with neoprene, or camping mat if there was enough room, with the leather over the top.
That is what I am trying to do at the moment. Actually trying to soften the aluminium with a torch and quentioning (sp) with cold water to soften so I can form it. Not doing very good at the moment.
