Agreed and fantastic idea.
In addition, the spell of Making should be VERY taxing on your sovereign.
He should HAVE to cast it from his capital.
He should HAVE to remain in his capital during the cast.
The cast should take an lengthy duration to cast. For a medium map, it should be no LESS than 30 turns.
EVERYONE should go to war with you as you cast the spell. I know this sounds grim, since you have been focusing on tech and not military might, but the result of casting the Spell of Making should be that you can survive until the spell is cast.
I like the ideas:
1 and 2 are related. As SOM immovilizes sov, it makes sense to cast it only in the capital. OR at lest, in one of the towers...
3 and 4 are related too. As the AI seem not reacting, 10 turns is enough. Working in the AI for casting the SOM, or trying to avoid the player casting it, will require more than 10 turns to play that part of the game.