Chastity and orgasm denial discussion

While I’m stuck in my cage, sometimes my wife listens to hypnosis tracks that make her feel the sensations we watch on screen. Other times, though, things get the best of her and she suddenly finds herself craving my cock. We’re trying to save money, so far we’ve set up a time release setup much like self-bondage: We freeze the key at the bottom of a water bottle. But when she’s sufficiently aroused, she can still hold that bottle under warm water to speed up melting, which somewhat undermines the goal. The idea is for her to have enough time to cool off and decide if she truly WANTS to let me out, rather than just having an immediate urge. So I’ve considered a few options, mostly ones outside our budget (mostly dedicated time release safes). My most frugal solution involves placing the key inside a locking thermos (perhaps still in its bottle), then freezing that entire thermos in another container… The thermos shouldn’t be too affected by temperature changes until you open the lid, and reaching that stage still requires her to do some work to thaw out the first key. I’m hoping someone else here might have a better idea… I’m all ears. We’re mainly just looking for something to ensure she can’t grab the key in under thirty minutes, so she’s tempted to touch herself or cool down before letting me go free. Any ideas?