I know, that this is a older post, but I made 3 shields this past Christmas for my nephews and here is how i made the boss. I had some pieces of 3/4" oak plywood, so i cut a 10" x 10" piece got a 3" hole saw and cut out the center. Then I got a wood rasp and beveled the circle. The first boss took most of an hour to make, and i thought there has to be an easier way. On the next one I put a couple clamps on two opposite corners of the steel to the plywood, and the job went much faster about 20 minutes. On the last one I clamped all four corners, and I was done in about 10 or 12 minutes. The shields themselves were from the Home Depot and were $5 table table tops.
Now if I were to do a full sized boss, I am sure, that I wouldn't use this way to do it. I think it would stretch the metal too much, but for children using wooden swords it should be fine.
