While briefly reading through the lab section, the Mental Construct flaw caught my eye. What level ritual do you think would be required to create such a lab?
I think that the laboratory that exists only in the imagination is one of the coolest ideas in the entire book.
I've no book here to try and calcualte the level from guidelines but I sugest that you look at the crown of Hermes in the laboratory items section and back-calculate the level from it.
That is a cool idea indeed. I wonder, speaking from experience, what happens when such a lab explodes?
Yair, who will only be getting his Covenants book in about two months... 
I, too, think this is one of the coolest ideas in the whole Covenants book. It takes the concept of the "memory palace" from mnemonic techniques, and gives it appropraite Hermetic significance. Here are the stats for the Crown of Hermes item, which apparently incorporates the effect:
Cr (In, Mu, Re) Me 50 (Base 35, +1 Touch, +2 Sun).
Wearing the Crown apparently gives the usual Warping point per year.
OTOH, having an "imaginary lab" means the magus has access to a fully-fitted lab which enemies or accidents cannot really destroy as long as the magus lives, takes no space, can follow the mage everywhere, and needs only time and meditation to make as efficient and richly outfitted as the mage desires.
Alas, it is only really useful for inventing spells or learning them from Lab Texts , or translating Lab Texts (but you only need the book). I'd guess that every sufficiently powerful and rich magus would prefer to have a "backup" second lab in one's head for spell research. Too bad it can't be used for Longevity Rituals and Vis Extraction (even if I have some qualms about the former, at least for the mage oneself: creating his or her own Longevity Ritual is after all an intensely personal task: even if you need physical equipment for the final task, performing the ritual, creating the formula should be possible).