we make a steel replacement cam cap, as we've had a few break also. the one that broke was the one right behind the waterpump impeller, right? and keep in mind... don't over tighten the bolts when putting it back together. use a torque wrench!
It looks to me like a small washer could be in order on the screws- they dig into the aluminum a bit. If that's where they are breaking- I should be taking mine apart!!
They break right where the countersink is for the bolt, it's a bit thin there. I don't know if a washer would help it or not, I kinda doubt it, the other caps have more "meat" to them and don't seem to be any problem.