ORIGINAL: jrcaster
For a 20C discharge LiPo pack, I wouldn't go with less than a 3000mah pack. The 2500mah pack you have listed would be getting close to minimum, plus your flight times would be rather low.
If you wanted to stay with the NiMh packs you would need to get two 6 cell packs and run them in series.
Be carefull with the hatch that you have made. Do you have any other supports inside the fuselage? I know you don't want to fly around and do a battery pack bomb drop.
OK, so looks like I will be going with my 2nd choice for the battery pack.
Don't know how to run batteries in series so I won't get into that.
The inside of the fuselage is plenty supported by the battery tray I made. The hatch itself is attached with a lip in the back and a 4-40 screw in the front. Also there are small supports inside the fuselage at the rear hatch area to prevent the hatch from being pushed in.