"Make an iron/wood hoop, call it a "ring", and take it whereever you need the effect enchanted upon it- quick and cheap (if physically vulnerable) "permanant" magic items. "
In theory yes, but you did a great job disproving that below. But at any rate, one could easily make a magic item that has a Range of Touch and a Target of Structure. That could be a ring, a plaque, a brick, or whatever.
"Can a ring move? Most magical effects don't. If cast on a wagon, is it mobile? If cast on a moving ship, would it travel with the vessel? Probably, possibly- but that's as part of a larger moving object.
To use Ring to define a (relatively) smaller, moving object, that moves the ring with it- I believe that's an error. Technically, I'd say the ring effects everything within it- not itself."
If it's attached to it, I don't see why it wouldn't move with it. If it was drawn on the ground around it then maybe not.
"Further, the definition of "Ring" states that the ring must be drawn while the spell is cast, using Concentration rolls- I would say that is a "do or die" moment for the enchantment. On a large enchantment, like a castle, to miss one Concentration roll (out of the many needed to create such a large ring) would waste the entire effort. (But I'm admittedly a hard-ass GM)"
I agree here, it's a cheaper way, not necesarily an easy or safer way.
"Unless the enchantment was placed upon the magi himself, "touch" was the limit, same as any spell. I haven't memorized the 5th ed rules- do they specifically address this issue?"
There is no more 'Self' range, it is now a 'Personal' range which affects only the caster and his clothes and Talisman (and vice versa a Talisman can have a 'Personal' range affect its creator). The range would thus have to be Touch. It is the Duration of Ring that would be used, as in whatever the Ring touches is affected, and it will remain in effect until the Ring is compromised.