How to navigate slide shows using one-handed keystrokes?

Here is one programmatic way: the combinations will be Option (Alt) + up / down arrows. Here is the code:

SetOptions[your-notebook, 
  NotebookEventActions -> {
    "DownArrowKeyDown" :> 
       If[MemberQ[CurrentValue["ModifierKeys"], "Option"],
          FrontEndExecute[
             {FrontEndToken[EvaluationNotebook[], "ScrollPageNext"]}
          ]
       ],
    "UpArrowKeyDown" :> 
       If[MemberQ[CurrentValue["ModifierKeys"], "Option"], 
          FrontEndExecute[
             {FrontEndToken[EvaluationNotebook[],"ScrollPagePrevious"]}
          ]
       ]
  }
]