Thanks again to all for the feedback!

i2i - I think you are right about using washers instead of the backplate, originally it was bigger to spread the weight and ensure the bag kept it's shape, but having reduced the size probably washers will suffice.

The screws have to pass through the handle strap in order to secure it, so they cannot go into the sidewalls of the part. Plus I need to keep the same 3mm wall thickness all round for cosmetic reasons.

Would it be helpful if I made up a 3D model of the exact size / shape? If so what kind of format would you guys require?

I'd love to hear from anyone who would like to take this job, I'm keen to get started on it.