Most modern LiPo batteries should fit snugly into the front battery 'cup' but will probably be too tall/thick to fit into the recess in the bulkhead, fitting a bulkhead from a re-release RC10 should solve that, as they are designed to suit modern batteries. Using two of the battery cups end-on-end with a shorty LiPo is a good solution too but you will have to trim a bit off each cup to allow them to sit closer together to suit the length of the battery.
If you go for the rear bulkhead option & a full length stick pack you will need to move the front battery cup forward slightly to accommodate the hardcase LiPo - which come up longer than the old Sub-C cell battery packs. I didn't bother re-drilling my chassis, I just stuck the battery cup down with double sided servo tape - with plenty of surface area they stick down well
