no "lederhose" is correct, "lederhosen" would mean at least two of them. German doesn't share the peculiar feature of English where you refer to a thing in plural if it is made to contain two legs.

Now for the fun part, lederhosen are mostly a Bavarian thing and AFAIK in Bavarian you'd say lederhos'n. This is still however the singular form. So it is still this lederhos'n vs. these trousers.

Disclaimer, my Bavarian is nearly non existent.