I would say (unless there are contrary examples I can't think of) that the usual practice is to change from plural to singular when you use a phrase like this adjectivally. E.g.:
This suitcase weighs 20 kilos. It's a 20-kilo suitcase.
That man is six feet tall. He's a six-foot-tall man.
This hotel was awarded four stars. It's a four-star hotel.
My watch cost 20 dollars. It's a 20-dollar watch. etc. etc.
Edit: Yep, what sound shift said, but more long-winded