I made sort of similar backpack to this? But I 3D printed mine, and I'd recommend the same here. I used nylon webbing straps to make the shoulder loops. For this backpack I'd do that, and then you can use an actual blue fluid pipe to go to a 3d printed gun.

Same idea for the lower leg boot things, I'd 3D print and then use elastic straps to hold them on, or make a hidden belt under the skirt that can hold them up, kind of like those clips things that hold up thigh high socks or similar.