A 'trouser' in singular would refer to only one of the legs of the trousers.
Since it has two legs, trousers would always be in the plural if you're talking about both of the pant legs.
I'm not so sure about the 'means.' In my opinion, I guess that since 'means' by itself is a plural noun, a singular form doesn't exist without having to change the meaning.
I apologize if I just confused you.