How can an NPC disguise themselves from the party for an extended period?

I have an upcoming storyline where I would like an NPC to hide their identity to gain the trust of the party. The nature of the ruse will involve them traveling with the party for an extended period (days or weeks) so I need something with a long duration.

Effects similar to alter self would be ideal but anything I can find lacks the required duration for my purpose. The Hat of Disguise, Greater modified to be continuous (double the cost) is the best solution I have found but is very expensive at 24,000gp and not something I want the party to potentially have access to.

Currently the PCs are 6th level and will be 7th or 8th when this storyline occurs. The party have a wizard that casts detect magic on everything he sees, a paladin that uses detect evil on every important NPC, and a druid with extremely high perception, I need to be able to fool all of them. They also have a bard and a barbarian but they are easier to trick.

Magic aura and undetectable alignment will get me some of the way there but suggestions that don’t involve so many resources are appreciated.

How can an NPC hide their identify from the PCs for an extended period of time?