A few comments-


Before buying a clamp on canti boss set I would do a lot of home work WRT how wide apart the bosses will end up at AND how wide you want them to really be. I've installed a bunch of bosses over the years as well as repaired hundreds of bikes with them. Many (most all) current (as of the 1990s) cantis want a wider boss spread then what some of the images will get you. For best braking power this is very important. This is why when I do a canti install I require the wheels and the brakes to be in my hands so I can mock up their fit. Often I find I need to augment the canti base to better both fit the stay/blade and also end yo with the best width apart.


As to stay diameter requirement, no. But there is a range of effect at play. Obviously the skinnier the stay the more spreading flex there will be. To counter this iy is easy to fab up a post to post brace, much like was done in the earlier days of MtBs. Once I felt the need to add a half sleeve onto the stay that helped spread the support beyond the bosses base.


Hopefully who ever you chose to do this addition is well versed in canti set up and not just a guy with a torch. Andy.