I think I'm ok with permanent chains.
I don't much care for making them permanent like the original spell, since it seems a bit powerful for a 6th/7th level spell.
The effective SRD spells to permanently trap beings are 8th+ level -
binding,
imprisonment and
trap the soul.
Admittedly
lesser planar binding is only 5th level and the bound creature could in principle remains trapped indefinitely, but that's a lot easier to escape from - a simple
dimension door or spell resistance check is enough.
I'd rather have the chains just last a bit longer than the cage, say an extra week or day per level (e.g. chain duration of 2 weeks/level or 8 days/level), and maybe allow them to be made permanent with a
permanency spell (but only the chains, not the cage).
Radius up to 2 ft per level?
The original has 3 ft. per level.
How about we average the two and call it 5 ft. per 2 levels, then it'll have a nice match in number of squares?
I don't think we've really talked about summon coven for this much. But something that allows an increase of effective CL in order to bind really nasty critters is what I'm thinking.
The original placed some limits on
summon coven - max duration 4 years, unable to increase max. diameter of circle, max chain strength 600 points, can make chains impervious to magic.
Hmm, impervious to magic - that reminds me, the original chains were only immune to fire and electricity. We've made them immune to any form of damage and not given them hit points (which to me is further reason not to make them permanent).
Also, it mentions that the
Day of Dread (a world-wide antimagic event) only interrupts the spell, and the subject it re-imprisoned when it expires. That suggests an
antimagic field would affect it in a similar fashion.
Do we want to reference any of that?