If a non caster uses an Action to cast Disguise Self while attuned to this hat, what is the Spell Save DC for the Investigation check DC?  Would it be just 8 + Proficiency bonus?

I know this is an old post, and you are correct, but: https://www.dndbeyond.com/sources/dmg/treasure#Spells

Spells

Some magic items allow the user to cast a spell from the item, often by expending charges from it. The spell is cast at the lowest possible spell and caster level, doesn’t expend any of the user’s spell slots, and requires no components unless the item’s description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration. Certain items make exceptions to these rules, changing the casting time, duration, or other parts of a spell.

A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don't have a spellcasting ability - perhaps you're a rogue with the Use Magic Device feature - your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.

Thank you for a graveyard reply to that question because I was about to literally look it up. :D

Best magic item.

At the very least one of the most useful.

Best version is a fedora of disguise self

Our group has a Mind Flayer, but she didn’t know how to describe her character, so I did some quick thinking and gave her one of these. 

Intelligence score of 20

This may be a dumb question, but do the effects of the spell continue after death of the wearer? I'm assuming since it just allows the wearer to cast the spell that the spell would end once the person dies, but I just want to clarify.

actually no. The casting time of disguise self is 1 hour, no concentration needed. It can be removed as an action, but unless that is specifies, it persists until its duration ends, technically even after death, unless the DM has specifically decided to rule otherwise.

Wow, just stumbled upon this item. Sure makes the warlock eldritch incantation Mask of Many Faces unnecessary.

Honestly it gives anyone else the equivalent, without having to be a warlock.

Quote from GreyAcumen >>

actually no. The casting time of disguise self is 1 hour, no concentration needed. It can be removed as an action, but unless that is specifies, it persists until its duration ends, technically even after death, unless the DM has specifically decided to rule otherwise.

 From the spell tooltip: casting time is 1 action and duration is 1 hour.

Quote from Daizuko >>

Wow, just stumbled upon this item. Sure makes the warlock eldritch incantation Mask of Many Faces unnecessary.

 Sure, but it uses up an attunement slot.  So a warlock could have that ability AND attune another thing.

id normally have it be Charisma fro this item because they have to pretend to be someone else, but that just me

Would be really useful tailing someone. Every time they go around a corner, change your appearance and it cuts down their chances of spotting the tail.

Would the spell go on forever if you never take of the hat? Also can the hat look like anything?

Spell still has duration, you at minumum would have to keep recasting.

How long does the spell last?

https://www.dndbeyond.com/spells/disguise-self

Hour.

Can I put one of these on a zombie I command and have him expend a charge to cast the spell?

Quote from Evil__Overlord >>

Best version is a fedora of disguise self

 "Technically it's a trilby"