In reply to Rhiannon yes, the midday sun varies in elevation through the year so if you want to know midday each day it would probably be simplest to just scratch a line, like in sundials. Whereas to me, Midday marked by a point (or notch or lightbox arrangement) implies an intention to mark midday on a particular day rather than every day.

But I'd go further in my "what's most practical is most likely" musings:
I'm thinking a shaft of light shining through a gap wouldn't be my timepiece of choice as if the gap was, say, 5 suns wide, the light shaft doesn't move very well and ought to be much narrower and it would be a real headache to fashion the gap so it worked every day. So shadows are best for marking the time year round - at midday the sun travels about one solar diameter per 2.5 minutes and the shadow reflects that rather sensitively all year round.