I have a couple of barber hones that I've turned into rounded-top hones for similar purposes. I just lapped them more on the sides than the middle until there was a roughly 1/8" hump running down the length of the hone. Their only use is honing warped razors, but they're good for that. A steady and consistent X pattern helps keep the wear distributed evenly.

You can also lap the sides of your favorite hone with a slightly rounded profile and hone on that. Unless you're using a combo hone or a glassstone, of course. If you want a high-grit ceramic "rod" then do this to the side of a Spyderco UF. Use a powered sander...