(I'd like to create a visual illusion of the wall there in the hole...as if it had never been opened.)
OOC
You are trying to create an image, so this is a Creo Imaginem effect. Going to the Imaginem guidelines on page 143, you can see that mimicking the specific wall requires a Perception + Finesse roll; AM will need to decide if it was high enough (hopefully, mimicking a wall that is right in front of you won't be hard).
It also says that an Individual amount is about the size of a human being - which seems about right to me, this is about the size of the illusion you want to create. This means that you can use the base guidelines given, and don't need to put in more power to create a bigger effect. If ArghMark decides that you need to increase size, then you'll need to add +1 magnitude to all the spell calculations below.
Moving on to the Creo Imaginem guidelines in the next page, I'd say you would want to use base 2 for two senses (sight + touch).
Now you need to add +1 magnitude to increase the range from Personal (the default) to Touch - you need to have the illusion form where your hands are waving, not on yourself (you can also use a larger range, like Voice - but that will just increase the spell's level needlessly).
The default duration is Momentary, which is hardly enough; you'll probably want to add +2 magnitudes to increase it to Sun. Higher durations are, of course, also possible.
The defailt target is Individual, which as I said above is enough.
Summing up, we've got a base spell level of 2, and +3 magnitudes, for spell level 5. (Further magnitudes will each add +5 levels, not +1 level - magnitudes add 5 levels past spell level 5.) The final spell is hence
Ilshana's Instant Wall
Range: Touch, Duration: Sun, Target: Individual
This spell creates the illusion of a wall segment. The illusion affects both sight and touch, but offers no resistance.
(base 2, +1 touch, +2 sun)
Now you need to cast it. That's handled on page 81. Your casting score is Creo + Imaginem + Stamina + Aure Modifier (whatever ArghMark says it is), which is about +12 for you (sans aura). Since you don't know the spell, you need to cast using Spontaneous Magic. There are two options - Non-Fatiguing has you divide your casting score by 5, which brings it to about 2, so you can't reach the desired spell level this way. You have to use the Faitguing option. This means rolling a stress dice, and adding your casting score, and then dividing by 2. If this comes out to 5 or more, you've cast the spell succesfully.
So, in practice, you need to:
* Make a stress roll, add your casting score (+12+aura), and divide the total by 2. Assuming the aura isn't too bad, and you don't botch, this should be about 5, enough to cast the spell.
* Make another stress roll (ArghMark can decide it's a simple roll, but that seems unlikely in the circumstances), and add your Perception + Finesse. If this is high enough (ArghMark's call), the wall appears good enough to fool (most?) people.
If any of these botches... well, that could be interesting...
The wall will probably suffice for cursory examination, but anyone trying to push through it will. It will fade at sunset.
Well, all except what is going on...
