Geezer here, mouthing off again.... this time about roller-nuts and sears.
Most of the strong medieval bows I have seen use a staghorn roller nut (there are a few that use brass) with an iron (or steel) sear. Payne-gallwey shows the sear-block as a pyramidal piece, fitted in the bottom of the nut. Egon Harmuth ("Die Armbrust, 1986") has radiographs that show a roughly pyramidal lower section, but usually the top of the sear-block is more or less cylindrical, and extends up through the top of the roller, to show a round spot between the lugs. Some very strong bows were fitted with slender iron/steel pins to reinforce the lugs... according to radiographs, those pins go almost through the roller to the bottom.
Later, multi-axle nuts have an iron/steel block fitted in the BACK of the roller-nut, rather than the bottom. That means the trigger system locks the nut from the back, rather than the bottom, which reduces the strain on the top edges of the nut-socket.
I have determined, after much experimentation, that moving the sear-plug back from dead-center bottom, about 1/4 inch substantially reduces nut-socket load. Of course that makes the geometry of the release a bit trickier.
Most of the roller-nuts I have seen range from a minimum of 30 mm to a maximum of perhaps 45 mm in diameter and an inch to two-inches in width (the Padre Island bow is the smallest lock I have seen, just a hair over 1 inch in diameter and the same in width)
If you anticipate going over 1000 lb draw, I suggest using brass with a steel sear, or possibly just machining the whole roller from steel. Medieval armbrusters generally didn't have the tools for that, but I'd say better safe than sorry. Needless to say, really strong bows will also need a strongly reinforced nut-socket for the roller to run in. Payne-Gallwey illustrates a strong steel socket used in later Flemish target bows. Again, that'll probably have to be machined.
Have fun stormin' da castle! Geezer.