"pencil, big clear plastic box..."
You have to get the voters to the pencil and big clear plastic box. This is a major problem in very rural voting districts on this side of the pond. In other parts of the world*, getting people past the various partisan citizens militias standing guard outside polling places is a problem.
Vote by mail has been proposed. It's actually a pretty secure system once all the bugs have been hammered out. Tampering with it doesn't scale well for national elections. But for local districts, fraud has occurred. Particularly one of the features often added to vote by mail: The volunteer ballot collection. A local GOP election win was invalidated for exactly this reason. The volunteers were picking up ballots and either revising or throwing out ballots from know Democrat leaning voters. And now that vote by mail is a hot topic again, volunteer ballot collection is a 'feature' that now the Democrats are pushing. They either forgot the last controversy. Or they think it's now their turn to fiddle with the results.
*Our dear mayor tried to invoke scenes of 'Federal troops invoking martial law and standing guard around polling places' come our November election day. No doubt all for the sound bite on a national radio show intended to trigger fear and doubt in the voting process. Because we have no polling places in our state (each state defines its own processes) and she damned well knows that. It's 100% vote by mail. But perhaps others don't know this.