I have a sling with leather pouch that just had bad holding ability so i soaked the pocket in water and put a rock in it tied a string around the pouch and let it dry.
when it was dry it had a nice cup  and no longer had holding probems.
I also find that after I did this with my tri-cord it now holds elipical ammo. I dried it around one of my six ounce egg sinkers
after I did this I relised I haven't seen that posted here
so thought I would thow in my two cents again

Seems like it should work. That's what we used to do on the lacrosse team - get the pocket of the lacrosse stick wet and force a ball into it, held there with a knife or ruler. The next day it would be formed and would hold the ball very well.

I've tried soaking, but it didn't work.  Tying the rock in sounds like it might work.  Maybe I'll be able to get the rocks to stay in while I'm trying the wide grip... instead of throwing rocks behind me into the apartments...