The Finale Copyist Text, Broadway Copyist Text and Jazz Text fonts by MakeMusic work with a variety of music scoring programs, including Finale and Sibelius. These fonts include the ability to surround text with handwritten looking boxes. To do this, special enclosure characters are available as part of the character set; a nice touch.
(use the regular, not extended versions of these music text fonts for this).
Finale Copyist Text and Broadway Copyist Text have a cool preassembled box which works really great with single numbers or letters. Simply type the tilde ~ character first, followed by a single digit or letter. They look like this:

For longer text strings, you could use standard open and closed bracket shapes. The main bracket shapes [ and ] produce just what you would expect. Shift-Open-Bracket and Shift-Closed-Bracket { and } produce a wider version of the open and closed bracket character.
However, if you just type either lower case or shifted open bracket followed by two or more letters or numbers and a closed bracket, you’ll see a separation of the enclosure:

more >> “Boxing Clever : Handwritten Font Enclosures for Music Notation”