Geezer here, concerning medieval/renaissance sights.
I've seen two types of sights on medieval crossbows. The big siege-bows used in competition in San Marino and Gubbio, use an off-set peep and aperture sight. The peep is offset, at the rear, somewhat behind the lock. The fore-sight is a notched aperture-plate mounted just behind (and to the side) the prod.
Renaissance gentlemen's bows use a centered folding rear-sight (with adjustable sight-plate), without any sort of locking mechanism. The point of your bolt serves as a front-sight. The sight, which is designed to fold forward, leans slightly back in service. The sight's weight will keep it upright, unless you're shooting substantially downhill. Presumably, it will fall forward on recoil, but that's all to the good. These bows invariably require a spanning device, usually gafa or cranequin, so a tall sight would be in the way for loading.
My SCA customers would rather have a sight that locks in place, thanks to light weight bows and competitions that require very high rates of fire, but in fact, all the folding sights I have seen have no lock other than gravity.
The 'diopter' sight, from Egon Harmuth's "Die Armbrust" as submitted by Mac, might make a useful alternative. I have actually seen a bow mounted with this sight... don't recall if it was in London or Vienna. Probably Vienna, at the Hofburg museum. One could experiment with placement of the little loops by taping them in place till you arrive at good solutions. Then a loop could be fabricated from a screw-in brass cup-hook for an authentic appearance.
Anyway, that's what I know about period sights. Geezer