It's best to decide what style/size battery pack you intend to use before you join the fuselage halves. That way you can cut out the battery area to fit while it's easy to do so.
Of course, this idea doesn't help if you've already glued it together.
I used a Kokam 3S 1500 LiPo in mine, and it's the perfect pack for this bird, IMO. It fits snugly into the battery slot, but only after you open the slot up a bit to get it to slide forward enough to balance with no clay. But it WILL balance...mine balances perfectly with the pack sticking out clear of the slot by about 1/2" inch or so.
Here's a trick I used to hog out the foam in the top of the slot a bit more after I'd already finished the build (I had opened the slot up some before fuse assembly, but it wasn't quite tall enough).
Take an old butter knife and bend the blade to about a 30-40 degree angle to the handle. Attach a strip of 60 grit sandpaper to the blade with a bit of rubber cement or spray glue and use it to sand out the slot to fit your pack. You could also add sandpaper on the bottom of the blade too, for the bottom of the slot, if necessary. Works great.