Have you tried inserting it as a symbol?
http://www.imsc.res.in/Computer/symbols-letter.pdf
Perhaps \boxempty (table 24)
See of the package is predefined...or check whether you need a math package (texycomp??)
An excerpt from the book
Most, but not all, of the symbols in this section are math-mode only. That is, they yield a “Missing $
inserted” error message if not used within $. . .$, \[. . .\], or another math-mode environment. Operators
marked as “variable-sized” are taller in displayed formulas, shorter in in-text formulas, and possibly shorter still
when used in various levels of superscripts or subscripts.