When I was making holsters and sheaths I used a contact cement. I think it was Pliobond. Coat both surfaces, let it dry a bit, carefully align the parts, and pound the heck out of them with a mallet or hammer to force them together. Be sure to carefully align the parts since they will stick to each other as soon as you touch them to each other.
There might be something new out there, but I don't know since I haven't made leather items in about 20 years or so.