An item that allows Theraphosa to assume human form at will? Do you want me to do it with my eyes closed, one arm behind my back, or both? 
Fiona already has a spell that does that (The Beast Remade...no offence, Thera, lass). Muto Animal(Corpus) level 25.
Let's see. Base 10 ("Change an animal into a human"), with a free requisite. The spell is R: Touch, D: Sun, T: Individual. Let's change that to D: Concentration and let the item maintain concentration for you, give you a little more flexibility as to when you can change forms. So that gives us a final Spell Level of 20.
Let's go ahead and give it 6 uses a day (+3) in case Thera gets bored. Her Magic Might is 15, so we need to dump 8 points into the effect to give it +16 Penetration. Concentration is +5. That gives us a final Effect Level of 20 + 3 + 8 + 5 = 36. Let's go ahead and make it so only Thera can use it (+3) and raise the uses a day to 12 (for an additional +1), making it a nice even 40 on the effect level.
Fiona's Lab Total is Int 1 + Magic Theory 6 + Mu 17 + An/Co 7 + 7 + Aura 5 + Inventive Genius 3 + Thera's Int 1 + Thera's MT (Enchanting) 6 + Paul's help 4 + Sheelagh's Int 0 + Sheelagh's Magic Theory 2 + Sheelagh's Inventive Genius 3 + Lab Bonuses 3 + Similar Spell Bonus 5 = 70, so we need to come up with another 15 points for the lab total. If we make the item jewelry or clothing, that's +4...I'm thinking a ring, bracelet, or a necklace/choker of some kind. Magnetite adds +3 for Animal, but she only gets a max of +6 because of her Magic Theory, soo...jewelry made of magnetite gives us a +6 Shape & Material bonus, which brings us to 76, 4 points to go. If she Experiments, that adds + 3 + 3 for Sheelagh + simple die = at least 7, so 83 plus.
She can take one more lab assistants, if we'd rather not Experiment.
Or, we can (blech) keep it Sun duration, making the Spell Level 25. Drop the Effect Frequency back down to twice a day, lowering the Effect Level by 3, add Environmental Trigger (Sunrise/Sunset) and remove the Maintains Concentration, for a net reduction of -2. That lowers the Effect Level to 35, and allows us to make the item in one season without experimenting. Which is boring, but whatevs. 