For a long time, I used Jansport packs, and they all had side pockets that fit the Nalgene quart bottles perfectly, so went with it. When I got into building packs, not just modifying them, thought of the Mountain Club frames used to carry supplies up to mountain huts. They had all the weight strapped to the very top of the frame. Since water is the heaviest item, switched from cylindrical to short boxy shaped quart bottles, and put two of them side-by-side and top dead center standing upright in inner pockets just under the pack top. On the right and left of the bottles were narrow inner pockets for smaller stuff.
Of course this had to be at least a 2/3 panel loader to zip down and get at the water bottles and other stuff. Had gotten to like the panel loaders with the Jansports. With water resistant zips and flap covers, they did not leak as often believed, and MYOG made it possible to build the pockets inside at the top.
But I finally realized that even packs without frames and side-arms are much more comfortable if they can rotate up and down with the hip belt while walking. But putting most of the weight up top made such packs top heavy and tippy when the hips rotated. So the bottles went to lower side pockets where they lowered the center of gravity. So with or without side-arms,, all the more recent packs have had the bottles on the sides. Not zip pockets, just tight cylindrical pockets with only lacing on the bottom so they can drain with leakage or rain.
The schemes for allowing the belts to rotate have varied from just the common one of slipping the belt though an envelope and held with velcro, to a recessed bolt that allowed full rotation. (I found that the belt only had to rotate a little bit, and chucked the hardware, which also wore out too often). But like an airplane propeller, they all rotated much better with the lowered center of gravity, which could be accomplished only with lowering the bottles. The rotation point worked best just above hip level at the small of the back.
I’ve often had to bushwhack on long trips due to huge rockslides and such, and the side bottles have not been a problem, or even noticed. Could be that is because the packs are not terribly wide, just wide enough to fit stuff sacks on top of each other. So the bottles don’t get in the way or bump into things.
So I’ve gone almost full circle back to conventional designs, except that the next pack is going to have the side arms back, but with a lowered center of gravity.
Given the moderator’s love for pictures, here are a bunch of older side-arm packs, posed for a photo:
