As to peeps that aren't quite peeps. There is one that takes a bit of practice, that is normally placed at the normal rear sight location.
It is simply a U shape and used with a bead that is properly sized to sit in the bottom of the U, with equal distance side to side and
on the bottom of the U up to the bottom of the bead.
Sighting is virtually identical to a peep sight, however there is not top- the top is open to meed regulations. It is a cheater that is not
cheating for use in chunk matches.
Kinda like this, but with straight vertical sides - a larger, in proportion bead would be easier to use, just like with normal peep sights.
A full or almost full buckhorn rear sight can also be used as a peep, if set up that way.
Of course, if you want to get serious about apertures. Here's my AJ Parker with German lens adjustable aperture.