Here is my suggestion.

Use Window's CharacterMap with the same font as your dialog use.
You could look in your resource file for that purpose.

"MS Dialog" is a good candidate for a unicode dialog.
The character 0xAF looks OK (reverse undersore).
Other characters may be good as well.
Like Arial unicode 0x2550 (Double lines) or 0x2014 (line centered).

TCHAR szLine[BUF_SIZE];

for(int i=0; i < BUF_SIZE; i++)
   szLine[i] = 0xAF;

szLine[BUF_SIZE-1] = 0;

That's it !

____
Jean