Help request: Hardening leather

I wanted to make a holster for a tool I use at work and was hoping to have it be a little stiffer than normal. I tried boiling water. And the damn thing shrank in three dimensions to about 1/2 to 2/3 original size (I haven’t mic’d it). It’s hard as a rock though. Much harder than I wanted.

So what are the tips and tricks I need to make some 5-7 veg tan tougher and slightly stiffer than normal.

https://imgur.com/a/xUFEVJN