I don't think you would even need to make a groove - just rough up the cast iron a bit first. When I epoxy hardened steel (or carbide) I also rough it up a bit with whatever it takes, sometimes a diamond burr on a Dremel. I heard that Robust used to use such a compound to glue the hardened rod to the tops of their steel rests but started tack welding when some popped off when dropped. They still use some kind of compound to fill in the gaps on the sides after welding.
I'm not stumping for Robust but as a satisfied customer I highly recommend their comfort rests and low profile rests for smaller things. They are the only ones I use now except for occasionally using a special rest such as a box rest. (And their replacement warranty is lifetime, no questions asked - problem, send it back.)
JKJ