In order to do this, there will be a lot intricate cuts, and bending of the shoulder piece. The back pack will be untouched. However, I'm also shortening the shoulder piece so eliminating the self choke Sticky out too far front bit.
Here's what I plan to do...
Using cosplay foam, I've made a rough template of the two components (and when I say rough, I mean it!!) as I'd rather use this and make a mistake, than botch the job on the actual shoulder piece.

&

Here's where I'm going to make the cuts & Bends...
First thing is figure out where I'm lowering the piece over my shoulders to stop it moving backwards or forwards and keep it in place while walking around...

then here's where I'm making the cuts...

Using ducktake to keep everything in place (Don't use glue as you'll need to remove for the template on the real parts) I calculated the angle and stuck the rear piece under the top of the back pack...

I did tell you that the template was a rough (yet to scale) bit of work!
Next I had to figure out how I was going to do the angled piece that reinforces the joint on the lid and backpack...

I'll keep you posted. This shouldn't take long at all as I'm happy with the overall look and feel of the template and best of all, I'm managed to lower the toilet lid, and make it far shorter so this will benefit movement of the helmet as well as what I can see.
Win Win and all that dealy kinda stuff.
Also to hide the joints I'll be using Mylar Chrome tape (ebay - £4.30 a roll of 10M x 10mm)