A friend and I were discussing this very matter on the hill a couple of weekends back. What i have seen blokes do in the past is buy cheap thin jandals and cut them to fit. This adds extra cushioning.
Also, have never seen these but in the 1960's there were a type of soft plastic innersoles available that had ridges on the top for grip and slots on the underside for ventilation, very popular with cullers. I think that Jandals with the corrigated sole would be a good start.
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