Options: every rank in spell mastery adds 10lbs, even 50 lbs depending on how strong you want illusions to be.
have the illusion cast on something, requiring 1-10lbs per pick in the illusion. If the object is stuck/broken (or moved?), the illusion loses a pick/is dispelled. Finding the object would take some luck, good intuition, detect illusion spell, illusion sight, etc.