What about the "trouser press"? Shouldn't it be plural according
to Quirk [A Comprehensive Grammar of the English Language, Longman,
1985]? Or is that form in use because it existed before the transition
Trouser press seems to follow the traditional rule of a
singular adjective. Still, trousers is a word that doesn't
normally have a singular form, like pants, shorts, slacks, and jeansbut
look how inconsistent: we have pants hangers and slack
hangers. We'd say Put your sweaters in the sweater drawer
and your shorts in the shorts drawer.
Tell your students that the English language is not always consistent.
There's a rule for this feature of noun modifiers, but the rule is changing,
as language does. You might remind them of the inconsistencies of English
spelling and pronunciation, too, which they have no doubt already noticed.