According to my patented n00b brand calculations, it checks out! Although you should remember the size qualification if you start trying to do this to the vatican or something. Of course, that would come with a different set of problems.
Well size modifires aren't added, so currently it's a standard size Structure.
Well a ritual is quite expensive as the current version is free. The magus I designed this spell for (not my own), is a Merinita so Until is certainly an interesting duration. Currently it's not used as a spell in our campaign, but we made a lesser enchantment for this effect. It is used with good effect on both our towers each month (after seeing the space needed for a 5th ed lab we had to cast a second CtMT to find living space).
If you add another magnitude you can probably distort the sounds form the structure so that they seem natural as well.
Not so free as you might think.
Now the magus who created this is slaved to casting it every moon. This is 3 or 4 castings per season, and who will cast it when he is off adventuring for a season.
With the year or until duration (and as he is Merinita, until has a distinct advantage here) the amount of burden on the magus is smaller, and the covenant only has to cough up 5 pawns of vis per casting. Until can be made to be a reasonably rare event (what was the quote from MacBeth?) so that it ends up only being cast 3 or 4 times in an entire campaign.
As this is a spring covenant, now 7 years from founding, we all feel that the 3 or 4 vis (can't recall precisely) used to make the item, and the couple of minutes it takes for one of us to use this item (two times) once a month is considerably less costly than a ritual. We assume, quite safely we feel, that there will allways be at least one or two magi present at the covenant.
It also may have been at the limits of the Merinitas abilities at the time (2 years since founding) to manufacture this item which produced the effect at moon duration.
Our ambition is to eventually make this effect as a continnual invested device, I think.
Except that it does allow the magus to cast it at his/her current level, while also laying the ground work for the development of a more robust version (duration: year) when the magus's art scores are sufficient to allow it.
The best choice might be to make it as a constant effect item. This wouldn't much more difficult than duration moon (serf's parma, 2 levels more?), you'd never have to worry about it again, and it would have a lower vis cost than casting the duration year spell even once.