Maybe you could allow Foci as physical manifestations of the Demon? Like you don't have them when you don't have magic, but can summon/create them when you do? Kinda flavoured like the Blackstaff-summoning in Changes. I'd say it wuld take a full action, with a difficulty roll of some kind, and possibly a point of Mental Stress. Maybe Lore, against the difficulty of the number of Focus slots it would use. One could, of course, accept Sponsor Debt to aid in this. I'd also say that you'd have to lock down the Focus before creation, as normal - no switching them on the fly or between summonings!
That said, summonable Foci seems a little powerful, although I suppose taking an action to summon them, taking a point of Mental Stress and having to make a difficulty Roll would mitigate that. It would also make the Foci be a decent investment, which I like. This template is all about 'power for a price', after all. THey would, obviously, dissolve if you got rid of Magic, or, if you wanted to make them last a little longer, at the end of the scene.