I spey fish for Salmon and Steelhead up until three years ago I would carry two fly boxes and extra tippet inside the top half of my waders. My wading belt kept the from slipping down.
That was fine if I was going to be in a area not too far from my vehicle.
As you can tell I was very limited to what I could carry with me to the river.
I purchased an LL Bean Rapid River Vest a few years ago and things changed. I started carrying more because I was moving around alot more.
Walk a few miles up or down river from where I parked became the norm.
Some of the very fine features of the Rapid River Vest aside from the amount of equipment you could stuff into it are the shoulder straps.
Adjustable, wide, and padded for comfort. Once on I very rarely ever have to shift it and after a while I hardly know I have it on regardless of the weight I am carrying.
Another feature that really mades a difference is the padded wasite belt and lumbar support. It also acts as a wading belt if you so choose to.
Even when climbing or casting the vest doesn't shift or waddle back and forth.