I have to confess it was me that used a hole saw, I am aware it sounds awfully crude, but I kept the hole saw central to the filter chamber using a thick plywood disk the same diameter as the filter chamber, with a central hole the same diameter as the hole saw, and then very carefully removed the eccentric rich casting. I do not consider this to be the perfect solution, I am aware that there are far superior engineering solutions out there, but I was just able to use tools that I already had in my toolbox, the teeth of the hole saw were set just enough to remove enough metal, and it enabled me to use the new type (at that time) paper filters, which due to the rich casting I was unable to do before I took my hole saw to it.