Learn Danish: Uden etiketten er det næsten ingens madkasse. - Without the label, it is almost no one's lunchbox.

Ingens means "no one's" or "nobody's" and refers to a person, not a thing. Because it refers to people in general, it doesn't change based on the gender or number of the noun it owns (like madkasse). You just add -s to the pronoun ingen.

Why does the sentence use the indefinite madkasse instead of the definite madkassen, since we are talking about a specific lunchbox?

In Danish, whenever you use a genitive (possessive) word like ingens (nobody's), the noun that follows must always be in its indefinite form. So it is ingens madkasse, never ingens madkassen.

Why does the verb er come before the subject det in this sentence?

This is a classic example of the Danish V2 (verb-second) rule. Because the sentence starts with the prepositional phrase Uden etiketten (taking up the first position), the verb er must immediately follow in position two, pushing the subject det into the third position.

Could I use ikke nogens instead of ingens here?

Yes, you could say ... er det næsten ikke nogens madkasse. In everyday spoken Danish, it is actually very common to split ingen into ikke nogen (and ingens into ikke nogens). However, using ingens as a single word is grammatically correct and good to know, especially for written Danish.