Qood question.
there's a neat modeling pruduct called Milliput. It's a two-part epoxy that is like soft clay.
Like most epoxies, there's a separate catalyst (stick) and resin (stick). Cut off two equal little chunks and knead them together in your fingers. Shape the blob into what you main bag body looks like.
It hardens in a couple of hours and can be sanded with a nail shaping stick, and painted.
Add straps and what-not with aluminum foil or similar, painted.
I tend to have a photo of what I'm trying to make in front of me, rather than relying on what I think the subject looks like.
https://www.milliput.com/howto.html
Have fun,
Bill