I have been using my Tanker's leather shoulder holster lately while trapping to keep a fullsize pistol towards the top of my waders and hopefully out of the water. That would work with a climbing holster and I think be comfortable, but then again I started wearing one in the 80s on tanks, so I'm used to it.
Here it was today when my daughter and I were leaving the swamp after checking traps. Notice the sweat? That's where my waders come too and it's a lot easier to access in this holster than on my belt.