What ? !!! Me giving advice to the "tool man" . LOL !
Yeah, bought some little round end bits from McMaster. They go on the Dremel. You know how the stock Dremel router base accepts two steel rods for an edge guide ? Well, I rig up some really long rods in there instead. Radius caul is clamped down to a jig (well, I put two counter-sunk screw holes in all my cauls, so they can be bolted down- in order to mill the radius into them). Then like a little moving, milling compass, I rout that groove in there with the Dremel. BUT, I think the way I do it is too complicated.
What I should do is use that same Dremel router base like a little router table and make some kind of little piece that surrounds the ball-cutter bit most of the way, or half-way, so that just a portion of the ball cutter is sticking out. Then you just work your caul across it and it's got that "safety stop" to make the depth the same across.
That Stew-Mac article actually shows something like that, except more basic (they seem to rely on the ball-cutter bits shaft acting like a depth stop. I think that would allow too deep of a cut).
I guess I should also mention that my "stock" dremel router base has a base I made, which is larger than the stock one, plus it's square, so it is like a little router table when turned up-side-down.
Heck, this is the kind of thing I usually only share with certain people off the forum.