I've used various types in both the mill and the lathe.The simplest way to make one for the mill is by turning it from drill rod on the lathe.Turn the profile you want,then mill two or three flutes in it providing for some back angle,then harden and hand grind the back relief.The smaller diameter means you can spin it faster and reduce the chatter.If the profile is fairly large or radical then removing as much material first by normal milling is best.
I've also made form cutters for the fly cutter,just did a layout and gorund to the line finishing up with hones.Still kept the diameter as small as possible.On those it helps to chuck the fly cutter in the lathe and clamp a machinists scale to the toolpost with it's edge 90* to the cutters axis.Move the scale's edge along the profile you have ground and rotate the cutter in the chuck checking the back clearence at various points on the profile.
Profile cutters on the lathe I make out of A-2 flat stock that I bolt into a tool holder.I layout,mill and file to shape,then heat treat and surface grind the top to sharpen.I've made these up to 3" wide and 1-1/2" deep,still hogging off the majority before using the form tool to finish at the lowest rpm.
Radius form tools for the lathe are easy to make,just a square piece of A-2 clamped in the mill vise on a 5-7* angle and a endmill centered on the corner and plunged through is it,surface grind top to sharpen after hardeneing is the usual.
I just need one more tool,just one!