I may suggest to make it like a Dora's backpack, separated from the back. . .
The "harness" ( don't know what it's called on backpacks) will be used as supports for the Gundam Backpack, make things short, don't mount it directly on the back. . .buckles will help the supports also.
https://fbcdn-sphotos-h-a.akamaihd....765_627159374033192_2058589706464620297_n.jpg
Don't know if this help but here's a quick paint sketch of the backpack idea.
or if you can afford to buy a HG gundam kit, that would help alot to. . .