I would think a metallic security sticker with your wording about warranty voidance could be placed over the edge of the NUC chassis, like this:
And also, you might want to place small round stickers over the four chassis screws, like here:
While something like this would not guarantee the chassis wouldn't be opened, at least you would KNOW if it had been.
A Google search for the same thing brought me here....Thanks Lois.
In addition for the tamper proof sticker, I was also searching for a source of the skins/labels (to be placed on the top and sides of the unit) for branding (again, as you are doing). We are also using the NUC as a Linux based appliance which we developed. Do you mind sharing your source for these?