I suppose the first question is "how much penetration do we need?" Unfortunately we don't know the horse's exact Might, but Fray can probably have a guess based on the spells we've cast at it:
- Stephen's Hands of the Grasping Earth failed to penetrate. That had a penetration of 9+die, so the horse almost certainly has more than 10 Might. Which isn't exactly surprising.
- Fray's Shape of the Fearsome King succeeded with penetration of 24 + die
So its might is probably somewhere between 10 and 35 - and I'd guess based on its nature it's probably in the 20 - 30 region.
We might be able to bring down the penetration needed in the device by triggering it in a more powerful aura than the horse will be fighting in, but I'd rather leave that as a back-up plan in case we realise we've misjudged something.
So, we have two options - try to boost Fray's lab total to the desired level, or switch to an invested device.
Fray's base lab total I think is actually 69, as he also has a lab bonus of Items +1. I'd recommend using shape and material of Animal Hide (+7 turn into an appropriate animal) + cloak (+4, capped at +2) rather than bothering with the Mercury, but it doesn't make much difference (and I suppose the Mercury might be useful for other enchantments if we do go down the enchanted item route). That means +9 penetration (or possibly +8 depending on how you want to do rounding).
I agree your calculation of the vis to open (I think the clasp and the embedded mercury would only be tiny, for 6 and 5 pawns, which means he couldn't quite open it as a compound device at the moment - which does limit the number of other effects we could put in it. That's probably not an issue unless someone has a brilliant idea for what else we should put in, though).
I'd also suggest creating the item with Concentration duration and Item maintains concentration rather than Sun duration, as this won't warp the grog, but that still leaves the level at 30.
Switching to an invested device has the advantage it allows us to instill other effects later as well, to make our "lion" more powerful; however, a lot of these could probably also be done in separate devices unless there's a reason a liniked trigger is valuable (or we can't do those in one season either).
If you did do it as an invested device, you could get a total level of 46 in two seasons, which would give you +32 penetration, which is probably but not certainly enough.
Options to increase Fray's lab total include:
Lab assistants.
Fray has no leadership score, so he's limited to 1 assistant unless he gets a familiar or all of his assistants are Bonisagi, their familiars or their apprentices. Potential assistants ranked in order of utility rather than availability are:
- Viola: Int +4 + MT 8 + items speciality 1 + IG 3 = 16
- Stephen: Int 3 + MT 9 + IG 3 = 15
- Constantine: Int +1 + MT 6 + lab assistant speciality 1 = 8
- Cicero: Int 1 + MT 6 + lab assistant speciality 1 = 8
- Gregorius: Int 1 + MT 4 - Creative Block 3 =2 ; also causes your final lab total to be reduced by 50% (Weak Enchanter) and your project to cost a third more vis (Waster of Vis).
So in theory, having Stephen and Cicero would probably do the trick; however, asking a fellow Bonisagus to be your lab assistant is probably a bit gauche (plus Trogdor hasn't posted in a while now). An alternative would be Fray spending a season to get leadership 1 with an appropriate speciality, at which point he could use 2 assistants regardless of who they were. That does still requires the assistants' time, though (and you really don't want Gregorius, which means we're talking getting either Stephen or Viola involved to get the necessary lab total).
Experimentation
This would give +3 (IG) + Risk Modifier (max 2) + simple die, which probably isn't enough to make a difference on its own. I'm also always reluctant to experiment on projects involving vis usage due to the potential resource wastage.
Boosting his Scores
This has the advantage that it's useful to Fray anyway, and the project isn't immediately urgent.
We don't have a useful Muto book. We do have a Corpus and an Animal book; however, the Animal Summa is a bit rubbish (Q8), and you'd need to raise both to get a single point of lab total. The other option is boosting your Magic Theory - we have 4 tractatus on this, which with your affinity would get you to MT 7 in two seasons (for +2 lab total due to the increase in the shape and materials cap), or 6 xp under MT 8 in a year).
You might also be able to enchant (or otherwise acquire) an item that gave you a boost to your lab totals.
Again, none of these are going to make a big enough difference on their own.