Aussienerd, that is the ideal system if I had the right shape lipos, some good delrin plastic, and some sort of locking system would be nice. Although it might be easier to make it a parallel slider vs. adjacent straight in, for alignment reasons.
The packs I have are Hyperion 6s 45c 2200mah packs, they are 105 x 34 x 53mm dimensions, so they are pretty short and stubby, just plain small... good for 10 mins runtime on my track with still ~20% SOC left. These things are awesome, wicked power, good handling from the light weight, and surprising super cool temps.
http://www.aircraft-world.com/shopexd.asp?id=3097
I would think they would fit inside this dewalt case, but I will have to maybe check it out at a hardware store, do a rough fit measurement.