To quote an answer by Ben to How can I best display a blank space character?, these two underbracket characters are possibilities:
- “Open Box” (code point U+2423, looks like ‘␣’). This can be entered in HTML as
␣
,␣
and␣
. - “Bottom Square Bracket” (code point U+23B5, looks like ‘⎵’). This can be entered in HTML as
⎵
and⎵
.
You can see these characters in PDF files on Unicode.org here and here.