Noting click adjustments negates most or all open sights and only opens the door to actual match-type sights. Here is an AJ Parker aperture sight. The Iris would have to be replaced with an "open" sight allowed by the rules.
The adjustment feature of this sight, is 1/4 minute clicks.
The mounting plate is attached to the sight itself on the groove rail - by a quick release lever plus single screw. The top flat wouldhave to be drilled and tapped for the mounting. I have one other aperture sight that has a long arm for mounting/screwing to the side of an action or perhaps side flat of an octagonal barrel. It is a .22 aperture sight - with Mossberg stamped on it. Obviously it is for mounting to the side of a .22 rifle's action. I do not have a picture of that and my current computer does not talk to my camera- yet.