A flat pack will probably not work for you. If you DO have the radio Box then there is no room on the chassis. If you don't have the box there is no way to mount it. I would sugest a pyramid shaped pack that you can either mount on the front between the shocks or on the rear bumper.
The pack I have is only five small cells, in a pyramid shape. It was custom made and I got it from a member here on RCNT. It works wonders and fits great. You would probably be better of scouring TowerHobbies.com for a good shaped pack that will fit than looking for a direct fit. Just about any configuration should work, Just find out what your servos ar rated for and base you decision on those facts.