They usually do a lumbar pad which is a wedge of foam in the middle of the hipbelt as you probably know, the wedge is usually thicker at the bottom than at the top to transfer the weight into the back.

Such as

If that doesnt work try this link you will see what i mean,

http://www.aventurenordique.com/exped-lightning-60.html


Looking good though, where did you get the info about the shape of the hipbelt please?