Who reads Ancient Greek, and why use two characters? Just use ∟ or ⦜ (Unicode “Right Angle” (U+221F), respectively “Right Angle Variant With Square” (U+299C). Don’t confuse either with ⌞, “Bottom Left Corner” (U+231E))

Tau is a Greek character, which is why I jokingly suggested this. Also, the above _is_ a single character, however, your suggestion is much more practical. The one with the square makes more sense since it may be ambiguous otherwise (90 or 270 degrees)

long after edit: I guess it's not technically one character!