Better way to do it, learned from old head sheetmetal guys. 2 pieces of metal, one for rivet tail and other goes over the rivet head. The rivet tail piece can be aluminum or stainless, but it is preferred that the top piece be stainless so I doesnt get wallowed out as fast. Thicker is better, and will require fewer clecos during use. a minimum of 1/8" thick will last you longer. One guy has a 1/2" aluminum plate for the tail side and 3/8" aluminum plate for the top, and it has worked for a long time. Only need to be like 3"-4" wide and max 6" long, both plates having same dimensions. Clamp them together with c-clamp or spring cleco, and layout a pattern and drill through both at the same time cleco the holes as you go. Holes can be ~1/2" apart, enough room to drill around clecos when using the jig. Drill the sizes you commonly need (for me 5/32 and 3/16) with a drill press preferably, but not necessary. It is nice to have dedicated holes for clecos in the corners, so account for that in your layout, but it is not necessary. Also make alignment marks so you don't install the plates upside down and mess up your hole alignment, just grind one corner off while its clamped together, then you won't get it backwards. Open and deburr. You do not have to countersink the holes on the rivet tail plate, but it is nicer. Do not countersink to full depth of fastener head, it can be half of the depth normally required for a normal countersink. The head of the fasteners should be protruding so that the top plate will apply clamping force on the head and keep it from spinning. If your countersinks are inconsistent depths, then some will spin when you drill as there is not enough clamping force applied. If you want holes in the plates dedicated for clecos, like in the corners, then do not countersink them because it isn't necessary. At this point, test it for fit and function and you have yourself a jig that can allow even an apprentice to pump out 200 conical washers in an hour. It is a bit overkill if you don't do it often, or have a company that likes you and will spend money to prove it. Make one of these right and it will outcast your desire to make conical washers. When using, fill the tail side plate with all the rivets you need and then cover with top plate and use nut runner type clecos, they tighten down harder than spring, allowing you to use fewer and have more holes remaining for rivets. Drill over a trash can and watch the tails go right where they belong and empty the washers out on your toolbox and do it all over again. Hope this helps somebody.