Haven't used it in years, but best I recall, the yellow 3M Super Weatherstrip Adhesive used to stick about anything to anything (as long as both surfaces were non-greasy). It was referred to by racers as "gorilla snot", as far as I know long before there was Gorilla Glue.
https://www.3m.com/3M/en_US/company-...2907682&rt=rud
If the leather will just be a lining I think some of the above glue suggestions will work. If it needs strength as in a crossbreed holster I would use screws and nuts.
http://www.shopdiyholster.com/eyelets-posts-and-screws/