I made my mortising chisel from a file, to cut the slots for the tabs, which go all the way through the web to the barrel. I made a chisel with a radius just a little smaller than 3/8" to cut straight in at the ends of the pipes, and take out the wood between with a 1/4" gouge and a 1/4" flat chisel, but I have a very gentle radius on the flat chisel so that it peels the wood more controlably from the inlet.