The format does depend on how the machine shop likes to work. BiLateral (or equal tolerenced +/- dimensions) is what I like to give to prevent mental math on what the target is.
The real problem is that on paper, both conditions would be a bad design. You are not controlling the minimum...
Using a positioning fixture seems like the most practical, cheapest method. Maybe a rod that fits under the U shape. Some other hardened features that gudie position on the outside of the Magnet.
Maybe a plate with the profile of the magnet cut just right over the work piece. You could use...
Yes, hardened steel chain would last longer wear wise, but it would also rust more. Stainless steel has low carbon, which makes it less hardenable. It is the carbon in steel that reacts to form rust. In a water enviroment that is probably your biggest concern.
You could try a grease to...
