This is probably the easiest piece to make yourself. Heck, I even made one out of a vinyl placemat for my first suit that worked just fine (not too mention it was comfortable). Since it will be buried under your girth and ammo belts, not to mention your jet pack, you shouldn't have too work too much about it being perfect.
you know funny as this sounds I have one, it came with my armor an since Jango's armor does not use it. Contact RA Im sure they could sell you one. If I can find mine laying around somewhere I'll let you know and you can have it for the price of shipping.