Would it not still be lederhosen? Like trousers and not trouser.
Although there is a place called Lederhose, so my etymology might be wrong, but Google isnt giving any trouser based results for Lederhose.
Disclaimer, my German is nearly non existent.
If using the word in English texts “lederhosen” is in a sense correct, though, also when only referring to a single one. This is a case where the loan word was slightly adopted, no doubt to bring it more into line with the convention in the English language of using plural for trousers.
That convention doesn’t exist in German, except when using English loan words („Jeans“, „Shorts“, „Boxershorts“).
BTW the plural comes from each 'trouser' originally being separate and held at the waist with a belt, like chaps.
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.