The end plug was clay and the hole was the right size, but I had made it and then glued it in place. In hindsight that sounds really dumb

Oh and I used a visco fuse which obviously contains some Sulphur but theres such little contact between the sulphur and chlrate theres pretty much zero risk. As long as you dont store it too long I dont think youre in any danger especially if you know that there is a tiny chance of it going off and hold it pointing away from you.
Soz just saw that im a bit late but hey sumone might be interested.