alyssame Your spar key can be in a container with a seal. You can carry it with you and in an emergency your break the seal or the container.
If you prefer to have your key at home another option place the key in a envelop and close it. For example, write a word over the glued opening and take a picture of it. Even if you put a lot of effort into it, it will always look different than this when you do it again.
If you want a third instant for keeping you in your belt you could use a box with time lock. There exist one which has a timer over max. 365 days. Is mad out of metal and not easy to break open. If the battery dies you can connect it to a USB charger.
But a key holder is more than taking care you get the key only when is the right time. A KH support you in your doing and archiving your goals, looks after you that you are in good condition, mental and physical.