You have to hold it or brace it into shape as it cools..Some have had luck using a hairdryer to warm into shape, then putting it into the fridge to cool..sometimes using edge moulding and fiberglassing the interior with wooden earwell shapes will do it..my suggestion would be to sell that one and get a good kit or look for a DP/CA at a good price..unfortunaely, most of the old DP/S's and newer Rubies have that problem..
Check over on omegawing theres some templates there to cut out some blocks out of wood or mdf that they install inside the ear pieces to stop them reverting back to the warped shape