Letter-shape as Description

The Chicago Manual of Style, 7.63 (Letters as shapes.)

Letters that are used to represent shapes are capitalized and set in roman type (an S curve, an L-shaped room). Using a sans serif font in a serif context, as is sometimes done, does not necessarily aid comprehension and, unless the sans serif perfectly complements the serif, tends to look clumsy.

The indication is in the context. Fluent English speakers are well used to using the shape of letters metaphorically. A T-junction, a Y-splitter, a U-turn, an A-frame, etc are all familiar concepts to the average AE speaker at least.

If you want to denote that the letter is being used for its shape and not any inherent meaning, you can put it in quotes, but there really isn't a need.