How about thinking outside the "Tk box" and pre-format your text before you assign it to the text attribute:

$ perl -le '
  $left  = sprintf( ">>%-10s<<", "left"  );
  $right = sprintf( ">>%10s<<",  "right" );
  print "$left\n";
  print "$right\n";
'
__output__
>>left      <<

>>     right<<
[download]

Hope it helps!