In 1990, scholar Nicasio Urbina published a bibliography of literary criticism on Ernesto Sabato. It can be found in the Dec 1990 issue of Hispania, the journal of the American Association of Teachers of Spanish and Portuguese (AATSP). While it only covers scholarship until 1990, it may still be of use.
A list of books and articles about El túnel begins on p. 961.
The full text is available in JSTOR via the link below (you must be logged in with your Hunter NetID and password to access JSTOR from off campus).