Jonas Not sure invitation is the correct word. Reminder might be more appropriate. Some sort of purity ring in the place where it might actually make a difference.
Invitation seems like it would be more suited for an item that wants to be taken off. Which I guess an unlocked chastity belt could also serve as, sort of steel lingerie. A lot of chastity belts do look rather pretty, after all.
But I feel an unlocked chastity device might be more useful for boys than for girls, as the erection-restraining function is still in place even when the device is not secure, which will discourage at least some impure impulses. Without an erection getting restrained, it might really just be the world's most uncomfortable purity ring.
The boys' version is actually a thing, there are a few sex toy companies out there selling silicone penis sleeves (that just slip on with no locking feature whatsoever) as chastity devices. Often not suitable for long-term wear either as they lack urination holes.