What's the strongest way to bond a cable stop to an steel frame?
Don't use cable stops.  The biggest load those stops will ever see is when you grab your brakes hard for an emergency stop.  So you're likely to find out that whatever you did to bond the stops to the frame wasn't strong enough or fatigue-resistant enough at the absolute worst possible time.
And you'll never be able to know if your work is fatigue-resistant enough.  You can only find out that it wasn't - and you'll know that only after it fails.
Run the outer housing the entire length of the cable.  All you need then is a few tie wraps to hold the cable near the frame.