When pondering the Target: Container, I came across the Ars communities thoughts on adding a new Duration: Seal as an allowed Hermetic spell constraint. Its a very good idea thrashed out by these community members (Ezzelino, Xavi, Timothy Ferguson, Gerg, YR7, Cuchulainshound, MarioJPC, GribbletheMunchkin, Jabir, Tellus, Richard Love).
Seal – this duration lasts until either the seal on the object is broken, or the object is opened in another way; the seal is active until it is effectively bypassed. The seal duration would be valid when used on a flask sealed with lead, but the spell would end immediately if the flask was broken, even if the flasks lead seal was not damaged.
The target of the spell using a Seal duration must be contained by the seal, the container, or the seal itself. Thus the contents of a bottle are valid, but a the object within a room behind a sealed door are not.
The seal must be intended to hold the object closed in a permanent manner, and must be applied to an object which could reasonably be sealed. Closing a door, or locking a chest is not permanent enough to use the Seal duration. Typical seals are wax sealed bottles, sealed letters, doors sealed with clay.
When used in this manner the seal is inscribed and marked in detail when initially created. Seal is a base: +3 magnitude duration when used.
This is a Duration that would require a Hermetic breakthrough to utilise, as while it has a narrow range of applications it does also extend some durations far beyond normal hermetic limits.