I'm working on making a pack, and, frustratingly, I feel compelled to continuously amend the design during the construction process. One of my aims was to achieve better ventilation than some of the other packs I've owned. I hope to improve my comfort during hot summertime hikes.
I don't really like the trampoline-back solution because it requires a rigid frame. Also, I'd like to use it for trail running and a pack that sits pretty snugly against the back bounces less. My target weight for the pack is 15-18oz.
One of my initial ideas was to use a thin layer of very breathable and resilient "reticulated" foam on the back and in the straps (image below). It's made of urethane and often used for filtration. I acquired some that's blue and has ten cells per linear inch. It's very bouncy and a breeze goes right through it.
I also considered 3D spacer mesh, but the stuff I've used in the past (and the material I've seen in packs and shoes) is pretty thin (1/8") and too compressible. It just gets squashed flat in use. It doesn't actually create any air space.
I found, though, that giant spacer mesh, with very open, netting-like face fabrics, in thicknesses from 1/2" to 1" are used for seat padding in some European recumbent bicycles and in under-mattress pads on boats. So, I bought some for experimenting.
It is very airy, and I can see through it. It is about 1/2" thick. In the second photo the thickness is compared to some standard 3D spacer mesh from DIY Gear Supply. It is very firm and bouncy. The specs say an area of one square yard will support 375 lbs. When I sit on it on a linoleum floor my butt doesn't flatten it.
It isn't light (about 20 oz/yard), but, for my project, I probably would only need about two square feet of it (or about 4 oz), which is probably only a net increase of about two ounces over the standard spacer mesh and foam it would replace. It is allegedly much more resistant to compression-set than foams (this is one of the benefits in applications like recumbent bike seats). The surface is slightly abrasive, so I'll probably try putting a light knit fabric over it wherever it might be in contact with skin.
I thought some of the MYOG crowd might be interested to know that this stuff exists. I'll post photos of it in use once my pack project is finished.