Your existing char(' ' *ones(20,72)) gives you a 20 x 72 char array of blanks, and is what you probably need. But what you might be looking for instead is
repmat({''}, 20, 72)
This will be a 20 x 72 cell array, each entry of which is the empty string.
In MATLAB, strings are row vectors of char and the only way to have an array in which each entry is a string, is to use a cell array.
