I have the pack pretty much assembled! I have a troop here in a few hours so I'm going to wear it to try it out. If the pack isn't comfy due to strap placement or whatever, I'll be able to ditch it nearby without any issues.
First I bought some nylon straps:
https://www.amazon.com/gp/product/B07H1D15LZ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
I then used a regular backpack strap to get a rough idea of how long I needed it and where to put the buckle. After cutting and melting/sealing the ends, I put super glue on about 1" of the strap and folded it over and clamped it. I then used an old soldering iron to burn a hole in the strap for a 1/4"-20 bolt. The straps were then attached to the tank with washers and nylock nuts. I used the mounting position of the straps from a couple other builds but I think the top strap needs to go up and inwards a hair but we will see after I do some trooping. Easy enough to change in any case.
I then tried it on. Granted its without armor so I'm not sure how its going to actually sit.
After deeming it "good enough" I took a black paint pen and covered up some of the exposed unpainted screws. It needs more touchup and weathering, but for the purpose of having a complete pack to take on a test troop I'm super happy with how its looking.
...keep in mind that this is my "rough draft" pack that I'm planning on giving to a friend. My revised parts, strap locations, improved assembly methods, etc. will all go into a second, more refined pack that I plan on keeping. I'll do a build on it as well and use those photos on my Thingiverse after I release the files. I have other projects and work and real life to take care of, but I suspect files will be available very soon for anyone else wanting to use my parts!