Hmm. This feels ok as an effect, but its mechanical implementation does not quite convince me.
First of all note that the level 3 MuCo guideline is about utterly changing the appearance or size of a person (or part thereof); I do not think changing a body part in such a way that it's disabled counts.
And here's the real deal. The spell effectively disables one body part - any part, in fact, that can be fuzzily defined as a "hole" (I'm not sure that an eye would count as one). My first reaction would be to say this is Perdo, and the "sealing" is merely a cosmetic effect. Compare to Twist of the Tongue: which does not destroy the tongue but merely twists it, thus destroying the ability to speak of the target. To which you may counter: Curse of the Unruly Tongue achieves something similar (but lesser in effect) through Rego... hmm. What can I say? It just does not feel Muto-ish.
Furthermore, from the examples in the corebook, it's an extremely versatile effect - existing examples of both Perdo and Rego magics target one specific body part/function: a hand, the eyes, the tongue. In fact, for Perdo different body parts/functions have different Base levels!
Hmmm. I'd actually allow the effect as described as a custom Perdo Corpus guideline. Base ... 15 maybe? This would make the final spell PeCo30 (Base 15, +2 Voice, +1 Diam). No research/breakthroughs needed. By comparison, making a person blind or deaf is Base 20, though since it's a "natural" change, D:Inst suffices, for a final Level still 30. You need two castings of this spell to cause serious trouble to the target rather than one, but its flexibility would roughly balance that, I'd say,
Finally, I'm not sure what the spell name is supposed to be. In Latin, it means "to mark/marking the body of the aperture". In Ars Magica, spells typically have English names like ... Sealing the Body's Gate. If you want to say in Latin "To seal/sealing the/a body's aperture" you would say something like "Foramen Corporis Sigillare". If you want to use the first person, present tense of the verb (after all, that's how the Techniques are named) "I seal ..." would be "Foramen Corporis Sigillo".